Include requirements.txt in the sdist

[musicinmybrain]

This fixes the problem of the sdist, as currently published on PyPI, not being usable because setup.py reads requirements from requirements.txt, but requirements.txt is not included.

Before this PR:

$ gh repo clone berenslab/MorphoPy
$ cd MorphoPy
$ python3 -m build
[…]
FileNotFoundError: [Errno 2] No such file or directory: 'requirements.txt'

ERROR Backend subprocess exited when trying to invoke get_requires_for_build_wheel

After this PR:

$ python3 -m build
[…]
Successfully built morphopy-0.7.2.tar.gz and morphopy-0.7.2-py3-none-any.whl
$ tar -tzf dist/morphopy-0.7.2.tar.gz
morphopy-0.7.2/
morphopy-0.7.2/LICENSE
morphopy-0.7.2/MANIFEST.in
[…]
morphopy-0.7.2/requirements.txt
[…]