Support Python 3.9.2+ (#121)

Without this, an exception will be thrown `AttributeError: '_EnumDict' object has no attribute '_cls_name'`

Refs: python/cpython@aba12b6

This [line](https://github.com/python/cpython/blob/3.9/Lib/enum.py#L97) expects attribute `_cls_name`.

Author: shadchin

enumfields/enums.py

@@ -1,3 +1,4 @@
+import sys
 import inspect
 from enum import _EnumDict
 from enum import Enum as BaseEnum
@@ -15,6 +16,9 @@ def __new__(mcs, name, bases, attrs):
             if hasattr(attrs, '_member_names'):
                 attrs._member_names.remove('Labels')
 
+        if sys.version_info >= (3, 9, 2):
+            attrs._cls_name = name
+
         obj = BaseEnumMeta.__new__(mcs, name, bases, attrs)
         for m in obj:
             try: