Separate requirements files by supported python version

[nhoening]

The .txt files (at the least, .in files can probably stay generic) should be available for each Python version we support, as the set of available packages per Python version differs.

This section of the pip-tools Readme states this as good practice, as well.

This has some consequences for Github actions and the Makefile, e.g. for the latter: can we pick the right requirements folder automatically by knowing the current environment's Python version? (on Github Actions, we set the Python version explicitly)

[nhoening]

Solved by #776