Generate requirements.txt for building a tarball

[nirs]

We like to publish the project on pypy (see #189), without requiring pipenv, so people that do not use pipenv can install the project using pip.

Tasks:

1. Generate requirements.txt from Pipenv when running python setup.py sdist
2. Make sure the file is mentioned in the MANIFEST
3. Make sure git does not complain about the unknown file in "git status"

[nirs]

Mostly fixed in c27abbe, should be done with #261.