Enable flake8-bugbear, but disable most checks. (#9499)

* Adds B00 to ignored checks.
* Fixes remaining issues.

tests/unittest.py

@@ -140,7 +140,7 @@ class TestCase(unittest.TestCase):
             try:
                 self.assertEquals(attrs[key], getattr(obj, key))
             except AssertionError as e:
-                raise (type(e))(e.message + " for '.%s'" % key)
+                raise (type(e))("Assert error for '.{}':".format(key)) from e
 
     def assert_dict(self, required, actual):
         """Does a partial assert of a dict.