Use gdbpy_ref in py-prettyprint.c

Use gdbpy_ref in py-prettyprint.c

This changes some spots in py-prettyprint.c to use gdbpy_ref.  It also
changes push_dummy_python_frame to be a class, rather than having it
create a cleanup.

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

	* python/py-prettyprint.c (print_stack_unless_memory_error)
	(print_string_repr, print_children): Use gdbpy_ref.
	(dummy_python_frame): New class.
	(dummy_python_frame::dummy_python_frame): Rename from
	push_dummy_python_frame.
	(py_restore_tstate): Remove.