Adjust to enum changes in Python 3.11.0b3

There are two changes:

Changes in the actual code:

 - _member_names changed from a list to a dict in python/cpython#28907
    - we instance-check and remove by list-specific or dict-specific way

Change in the tests only:

 - accessing other enum members via instance attributes is no longer possible
    - we access them via the class instead
    - we leave the original test in a try-except block

Some of the Python enum changes might get reverted,
see python/cpython#93910
But the fix is backwards compatible.

Fixes googleapis#326

proto/enums.py

@@ -58,8 +58,11 @@ def __new__(mcls, name, bases, attrs):
         # In 3.7 onwards, we can define an _ignore_ attribute and do some
         # mucking around with that.
         if pb_options in attrs._member_names:
-            idx = attrs._member_names.index(pb_options)
-            attrs._member_names.pop(idx)
+            if isinstance(attrs._member_names, list):
+                idx = attrs._member_names.index(pb_options)
+                attrs._member_names.pop(idx)
+            else:  # Python 3.11.0b3
+                del attrs._member_names[pb_options]
 
         # Make the descriptor.
         enum_desc = descriptor_pb2.EnumDescriptorProto(

tests/test_enum_total_ordering.py

@@ -49,7 +49,11 @@ def test_total_ordering_w_other_enum_type():
 
     for item in enums_test.OtherEnum:
         assert not to_compare == item
-        assert to_compare.SOME_VALUE != item
+        assert type(to_compare).SOME_VALUE != item
+        try:
+            assert to_compare.SOME_VALUE != item
+        except AttributeError:  # Python 3.11.0b3
+            pass
         with pytest.raises(TypeError):
             assert not to_compare < item
         with pytest.raises(TypeError):