Add requirements.in to valid requirements files

[bpereto]

Add requirements.in to valid requirements files, not just requirements.txt

I keep the requirements in a requirements.in file and "compile" a requirements.txt through pip-compile (https://github.com/jazzband/pip-tools).

The workflow could be more simple than executing:

cp requirements.in requirements.in.txt
pip-upgrader requirements.in.txt
mv requirements.in.txt requirements.in
pip-compile requirements.in > requirements.txt