SCons: Fix Godot detection in custom modules logic
`exec()` was not a good idea as it assumes a certain type of `version.py` file
similar to Godot's own file, which is not always a reliable assumption (see
https://github.com/godotengine/godot/pull/43057#issuecomment-777632900).
Also restores Python 2 support for the 3.2 branch.
(cherry picked from commit 75910d1e9b37dc5647a9f8883c5a2268188a2423)