Unify pack file version and magic to avoid hardcoded literals.

`version.py` now always includes `patch` even for the first release in
a new stable branch (e.g. 3.2). The public name stays without the patch
number, but `Engine.get_version_info()` already included `patch == 0`,
and we can remove some extra handling of undefined `VERSION_PATCH` this
way.
Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>