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.