gdb libpython.py: Remove compatibility code (#105739)

Remove compatibility code for Python 2 and early Python 3 versions.

* Remove os_fsencode() reimplementation: use os.fsencode() directly.
  os.fsencode() was added to Python 3.2.
* Remove references to Python 2 and "Python 3": just say "Python".
* Remove outdated u'' string format: use '' instead.