Include .pyx files in source distribution

[irregulator]

This commit adds .pyx files in MANIFEST.in, thus includes Cython files in pystemd source distribution (sdist).

The rationale behind this change is based on the following:

• Cython documentation [1] suggests distributing "the generated .c files as well as your Cython sources so that users can install your module without needing to have Cython available." Distributing .c files should be considered a helpful addition to the source distribution, they do not actually replace the .pyx files.
• Including .pyx files in source distribution is in accordance with LGPL 2.1 terms [2] reading: "Source code" for a work means the preferred form of the work for making modifications to it.
• Including .pyx files in sdist is especially important for Debian packaging where we need to be sure all binaries are able to be built reproducibly from their source.

Additionally, current commit restructures setup.py external_modules by inversing their logic: if no pystemd/*.c files are found then cythonize is called on .pyx files. Both file types which will be anyway included in sdist.

[1]https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html#distributing-cython-modules [2]https://www.gnu.org/licenses/old-licenses/lgpl-2.1.html

[aleivag]

yeap, the reasons make sense, i was also planing to start including them, because they help with stacktraces and how cython reconstruct the frames on a exception (trace back the error to the pyx file).

i'll look at the details later today and merge

[facebook-github-bot]

@aleivag merged this pull request in facebookincubator/pystemd@a51e19e1abe65498f2d74aa540c7016716c3e846.