Change pspace_to_pspace_object to return a new reference

Change pspace_to_pspace_object to return a new reference

This changes pspace_to_pspace_object to return a new reference and
fixes up all the callers.

gdb/ChangeLog
2018-09-16  Tom Tromey  <tom@tromey.com>

	* python/py-inferior.c (infpy_get_progspace): Update.
	* python/python-internal.h (pspace_to_pspace_object): Change
	return type.
	* python/py-newobjfileevent.c
	(create_clear_objfiles_event_object): Update.
	* python/py-xmethods.c (gdbpy_get_matching_xmethod_workers):
	Update.
	* python/python.c (gdbpy_get_current_progspace): Update.
	(gdbpy_progspaces): Update.
	* python/py-progspace.c (pspace_to_pspace_object): Return a new
	reference.
	* python/py-objfile.c (objfpy_get_progspace): Update.
	* python/py-prettyprint.c (find_pretty_printer_from_progspace):
	Update.