Use gdbpy_enter in py-inferior.c

Use gdbpy_enter in py-inferior.c

Change py-inferior.c to use gdbpy_enter.

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

	* python/py-inferior.c (python_on_normal_stop, python_on_resume)
	(python_on_inferior_call_pre, python_on_inferior_call_post)
	(python_on_memory_change, python_on_register_change)
	(python_inferior_exit, python_new_objfile, add_thread_object)
	(delete_thread_object, py_free_inferior): Use gdbpy_enter.