python: Add Progspace.objfiles method

python: Add Progspace.objfiles method

This patch adds an objfiles method to the Progspace object, which
returns a sequence of the objfiles associated to that program space.  I
chose a method rather than a property for symmetry with gdb.objfiles().

gdb/ChangeLog:

	* python/py-progspace.c (PSPY_REQUIRE_VALID): New macro.
	(pspy_get_objfiles): New function.
	(progspace_object_methods): New.
	(pspace_object_type): Add tp_methods callback.
	* python/python-internal.h (build_objfiles_list): New
	declaration.
	* python/python.c (build_objfiles_list): New function.
	(gdbpy_objfiles): Implement using build_objfiles_list.
	* NEWS: Mention the Progspace.objfiles method.

gdb/doc/ChangeLog:

	* python.texi (Program Spaces In Python): Document the
	Progspace.objfiles method.
	(Objfiles In Python): Mention that gdb.objfiles() is identical
	to gdb.selected_inferior().progspace.objfiles().

gdb/testsuite/ChangeLog:

	* gdb.python/py-progspace.exp: Test the Progspace.objfiles
	method.