It's more robust to have the version defined statically in a header file,
rather than dynamically generating it via git using a git tag.  In case
git doesn't exist, or a different source control tool is used, it's
important to still have the uPy version number available.

The line in docs/conf.py with the release/version number was recently
changed and this patch makes the makeversionhdr.py script work again.

This script may be called by Windows IDEs (e.g. Visual Studio) and be passed
paths with backslashes.

Addresses issue #1420.

These scripts should run under as wide a range of Python versions as
possible.

Addresses issue #1285.

Also rename py-version.h to mpversion.h for consistency with mpconfig.h.