Minor Python simplifications

I noticed that a few "#if"s could be removed from the Python code.
This patch is the result.

gdb/ChangeLog
2020-11-02  Tom Tromey  <tromey@adacore.com>

	* python/python.c: Consolidate two HAVE_PYTHON blocks.
	(python_GdbModuleDef): Move earlier.  Now static.
	(do_start_initialization): Consolidate some IS_PY3K blocks.