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.
3 files changed