Added version_files attr to write version in python files

[mathieulongtin]

Not sure if you're interested, but I added an attributes to update the version.py files.

It's not exactly "pure" since it writes files, but it doesn't change anything to the original behavior.

[ehrenfeu]

I really liked setuptools-git-version since it is a nice and small package, but I actually do have the need for some functionality like this PR would add. For the time being, I migrated to the (much more complex) setuptools_scm which is recommended in the Single-sourcing the package version (last point at the very bottom) of the Python packaging user guide.