fix: enums initialization in PyPy

When removing the _pb_options name from the enum attributes,
enums initialization in PyPy should not raise exception.

Refs: #506

proto/enums.py

@@ -61,6 +61,8 @@ def __new__(mcls, name, bases, attrs):
             if isinstance(attrs._member_names, list):
                 idx = attrs._member_names.index(pb_options)
                 attrs._member_names.pop(idx)
+            elif isinstance(attrs._member_names, set):  # PyPy
+                attrs._member_names.discard(pb_options)
             else:  # Python 3.11.0b3
                 del attrs._member_names[pb_options]