API Core: fix pytype build (#6873)

* Run 'pytype' only over the 'google/' directory.

* Ignore 'pytype_output/' derived files.

* Remove spurious 'MutableMapping.register' call.

'pytype' chokes on it, but the 'Policy' class already derives from
'MutableMapping', so the call is a no-op.

* Silence deprecation spew during IAM unit tests.
diff --git a/setup.cfg b/setup.cfg
index 046436b..973b943 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -4,6 +4,6 @@
 [pytype]
 python_version = 3.6
 inputs =
-    .
+    google/
 exclude =
     tests/