bpo-9263: Use _PyObject_ASSERT() in gcmodule.c (GH-10112)

Replace assert() with _PyObject_ASSERT() in Modules/gcmodule.c
to dump the faulty object on assertion failure to ease debugging.

Fix also indentation of a large comment.

Initial patch written by David Malcolm.

Co-Authored-By: David Malcolm <dmalcolm@redhat.com>