This essentially reverts commit 866566a690b, which installed safeguards
against loading plpython2 and plpython3 into the same process. We don't
support plpython2 anymore, so this is obsolete.
The Python and PL/Python initialization now happens again in _PG_init()
rather than the first time a PL/Python call handler is invoked. (Often,
these will be very close together.)
I kept the separate PLy_initialize() function introduced by 866566a690b
to keep _PG_init() a bit modular.
(I'm looking at the some further work to allow multiple interpreters in
PL/Python, so I'm trying to clean up the initialization code a bit first.)