bpo-46756: Fix authorization check in urllib.request (GH-31353)

Fix a bug in urllib.request.HTTPPasswordMgr.find_user_password() and
urllib.request.HTTPPasswordMgrWithPriorAuth.is_authenticated() which
allowed to bypass authorization. For example, access to URI "example.org/foobar"
was allowed if the user was authorized for URI "example.org/foo".