`python setup.py install` doesn't install `snappy._snappy_cffi` module

Reproduce the failure with the listed Python 3.6 virtualenv steps, then compare python setup.py install with the working bdist_egg path. Inspect setup.py and the reported snappy._snappy_cffi, _snappy, and compiled extension files; done means a direct install allows import snappy without manually copying files.

Related: #78, #69

Repro

macOS 10.14.5, in a virtualenv with Python 3.6.9, python-snappy v0.5.4 (commit 602e9c10d743f71bef0bac5e4c4dffa17340d7b3)

1. git clone https://github.com/andrix/python-snappy
2. cd python-snappy
3. git checkout master
4. python setup.py build
5. python setup.py install

Testing in IPython fails with the following error:

In [1]: import snappy
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
~/Documents/python-snappy/snappy/snappy.py in <module>()
     47 try:
---> 48     from ._snappy import UncompressError, compress, decompress, \
     49                          isValidCompressed, uncompress, _crc32c

ModuleNotFoundError: No module named 'snappy._snappy'

During handling of the above exception, another exception occurred:

ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-1-710a7f18823c> in <module>()
----> 1 import snappy

~/Documents/python-snappy/snappy/__init__.py in <module>()
      1 from __future__ import absolute_import
      2
----> 3 from .snappy import (
      4         compress,
      5         decompress,

~/Documents/python-snappy/snappy/snappy.py in <module>()
     49                          isValidCompressed, uncompress, _crc32c
     50 except ImportError:
---> 51     from .snappy_cffi import UncompressError, compress, decompress, \
     52                              isValidCompressed, uncompress, _crc32c
     53

~/Documents/python-snappy/snappy/snappy_cffi.py in <module>()
      1 from __future__ import absolute_import
      2
----> 3 from ._snappy_cffi import ffi, lib
      4
      5 try:

ModuleNotFoundError: No module named 'snappy._snappy_cffi'

Workaround

1. python setup.py bdist_egg
2. cd dist
3. unzip python_snappy-0.5.4-py3.6-macosx-10.14-x86_64.egg
4. cd snappy
5. cp _snappy_cffi.abi3.so "${VIRTUAL_ENV}/lib/python3.6/site-packages/snappy/"

After performing the above steps, it is now possible to import snappy in a python shell without error.

(might also need to copy _snappy.py and _snappy.cpython-36m-darwin.so; not sure)

It appears that python setup.py install does not install these files.