Use correct CFBundleExecutable in the Info.plist for Python.app

The Python build machinery apparently does not use proper autoconfigury to
expand this Info.plist.in file, so can't use @PYTHONFRAMEWORK@ as for the
Info.plist for the framework itself, but have to hardcode LibreOfficePython.

As such I am not sure that Python's way of including an app bundle
inside a framework's Resources subtree is acceptable in the stricter
code signing and Gatekeeper rules that soon will be in effect. Will
see.

Change-Id: I1ef9e7b748d41ec4b32d80e721d5fba5e7a90d18