Introduce gdbpy_enter

Introduce gdbpy_enter

This introduces gdbpy_enter, a class that can be used to acquire and
release the Python GIL, and also set other Python-related globals used
by gdb.  ensure_python_env is rewritten in terms of this new class.

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

	* python/python.c (gdbpy_enter): New constructor.
	(~gdbpy_enter): New destructor.
	(restore_python_env, ensure_python_env): Rewrite.
	* python/python-internal.h (gdbpy_enter): New class.