bpo-30167: Prevent site.main() exception if PYTHONSTARTUP is set. (GH-6731)

bpo-30167: Prevent site.main() exception if PYTHONSTARTUP is set. (GH-6731)

Before Python 3.6, os.path.abspath(None) used to report an AttributeError which was properly caught inside site.abs_paths, making it ignore __main__, one of sys.modules, which has __file__ and __cached__ set to None. With 3.6, os.path.abspath(None) raises TypeError instead which site.abs_path was not expecting.  This resulted in an uncaught exception if a user had PYTHONSTARTUP set and the application called site.main() which a number of third-party programs do.