- Repair _reinstall_default_lookups to also flip the _extended flag

- Repair _reinstall_default_lookups to also flip the _extended flag
off again so that test fixtures setup/teardown instrumentation as
expected
- clean up test_extendedattr.py and fix it to no longer leak
itself outside by ensuring _reinstall_default_lookups is always called,
part of #3408
- Fixed bug where when using extended attribute instrumentation system,
the correct exception would not be raised when :func:`.class_mapper`
were called with an invalid input that also happened to not
be weak referencable, such as an integer.
fixes #3408