Does not install into a venv on Ubuntu 20.04

Start with README.md and reproduce the pip install in an Ubuntu 20.04 Python 3.9 virtual environment. Check the documented packages against the available libsystemd-dev package and inspect the C compilation failure involving systemd/reader.c. Done means the documented dependencies and installation path work in that environment.

The README.md says I can just apt install the headers. However, the only systemd header package available is libsystemd-dev. The others, like libsystemd-journal-dev and libsystemd-daemon-dev do not exist for 20.04, so I can't install them.

If I try to pip install into my venv anyway, I get a bunch of compilation errors (as I figured). They seem related to unicode. Anyway, here's the last lines showing the pip install crashing:

      cython_utility:179:25: warning: ‘PyUnicode_AsUnicode’ is deprecated [-Wdeprecated-declarations]
      In file included from /home/jecompt/.pyenv/versions/3.9.5/include/python3.9/unicodeobject.h:1026,
                       from /home/jecompt/.pyenv/versions/3.9.5/include/python3.9/Python.h:97,
                       from systemd/reader.c:23:
      /home/jecompt/.pyenv/versions/3.9.5/include/python3.9/cpython/unicodeobject.h:580:45: note: declared here
        580 | Py_DEPRECATED(3.3) PyAPI_FUNC(Py_UNICODE *) PyUnicode_AsUnicode(
            |                                             ^~~~~~~~~~~~~~~~~~~
      cython_utility:179:25: warning: ‘_PyUnicode_get_wstr_length’ is deprecated [-Wdeprecated-declarations]
      In file included from /home/jecompt/.pyenv/versions/3.9.5/include/python3.9/unicodeobject.h:1026,
                       from /home/jecompt/.pyenv/versions/3.9.5/include/python3.9/Python.h:97,
                       from systemd/reader.c:23:
      /home/jecompt/.pyenv/versions/3.9.5/include/python3.9/cpython/unicodeobject.h:446:26: note: declared here
        446 | static inline Py_ssize_t _PyUnicode_get_wstr_length(PyObject *op) {
            |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~
      error: command '/usr/bin/gcc' failed with exit code 1
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: legacy-install-failure

× Encountered error while trying to install package.
╰─> systemd