gdb/python: remove Python 2/3 compatibility macros

New in this version:

 - Rebase on master, fix a few more issues that appeared.

python-internal.h contains a number of macros that helped make the code
work with both Python 2 and 3.  Remove them and adjust the code to use
the Python 3 functions.

Change-Id: I99a3d80067fb2d65de4f69f6473ba6ffd16efb2d