libsystemd-dev needed also for installation via pip

Read the README's Installation and To build from source sections first. Update the Installation section to mention that pip installation requires the Debian libsystemd-dev package, then verify the dependency guidance is consistent with the existing source-build instructions.

Installing this package with:

pip install systemd-python

On a Debian oldstable (Bullseye), I got an error message very similar to the ones reported in #47 and #97:

pip install systemd-python
Collecting systemd-python
  Downloading systemd-python-235.tar.gz (61 kB)
     |████████████████████████████████| 61 kB 83 kB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_605170e387494e1a9bdc2c7ba6ddef4a/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_605170e387494e1a9bdc2c7ba6ddef4a/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-zqy_tu5b
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_605170e387494e1a9bdc2c7ba6ddef4a/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/10/9e/ab4458e00367223bda2dd7ccf0849a72235ee3e29b36dce732685d9b7ad9/systemd-python-235.tar.gz#sha256=4e57f39797fd5d9e2d22b8806a252d7c0106c936039d1e71c8c6b8008e695c0a (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  Downloading systemd-python-234.tar.gz (53 kB)
     |████████████████████████████████| 53 kB 496 kB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_fce97db8c3184d04b9266626a33ab67b/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_fce97db8c3184d04b9266626a33ab67b/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-2hsisnk0
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_fce97db8c3184d04b9266626a33ab67b/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/e8/a8/00ba0f605837a8f69523e6c3a4fb14675a6430c163f836540129c50b3aef/systemd-python-234.tar.gz#sha256=fd0e44bf70eadae45aadc292cb0a7eb5b0b6372cd1b391228047d33895db83e7 (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  Downloading systemd-python-233.tar.gz (49 kB)
     |████████████████████████████████| 49 kB 1.6 MB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_bb46b5166fe1470496a4586b7b1b3884/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_bb46b5166fe1470496a4586b7b1b3884/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-95vgxi6u
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_bb46b5166fe1470496a4586b7b1b3884/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/10/89/d66ae73bdbc2614e4f2e90ddf9ef80d22c28f3cd70071345c3640465c14c/systemd-python-233.tar.gz#sha256=06a3768b744884565d7961b44f4a51d164765bc59692f0f85ea6f11330a17da4 (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  Downloading systemd-python-232.tar.gz (49 kB)
     |████████████████████████████████| 49 kB 1.4 MB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_f10a8673949f4842a0bb53f7ea5fe779/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_f10a8673949f4842a0bb53f7ea5fe779/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-oql0295y
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_f10a8673949f4842a0bb53f7ea5fe779/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/87/a3/e73a8373d40b05a75bb34fdccae606ab4dc0c0743535289904f15eb624f5/systemd-python-232.tar.gz#sha256=e5d0fdbdf50e7d67c77e08bc7986ef41d98aef222896bc033c27477893c3d4ba (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
ERROR: Could not find a version that satisfies the requirement systemd-python
ERROR: No matching distribution found for systemd-python

Installing libsystemd-dev with apt install libsystemd-dev allowed me to install the package.

I would mention this also in the "Installation" section of the Readme (that is the section I read when I tried to install via pip...) and not only in the "To build from source" one.