Use gdbpy_enter in python.c

Use gdbpy_enter in python.c

This changes the last functions in python.c to use gdbpy_enter.  I
split gdbpy_finish_initialization into two functions in order to avoid
some "goto"s.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/python.c (python_command): Use gdbpy_enter, gdbpy_ref.
	(do_finish_initialization): New function.  Use gdbpy_ref.
	(gdbpy_finish_initialization): Use gdbpy_enter.  Call
	do_finish_initialization.