Make Python 2 and 3 versions parallel installable

Problem: Both Python 2 and 3 versions of pygobject share the same header and .pc file. Distros build them both and create a shared dev package ususally. This means distros can't update to 3.38 as long as they have to support a Python 2 version as well.

Possible solution: Add an "--alternate-dev" option to the Python 2 package so the .pc file and the include directory are something like "pygobject-3.0-py2" instead of "pygobject-3.0". This means the depending packages need to be patched, but that should only be a hand full.

Ideas welcome.