Remove deprecated ABCmeta-based ABC class

src/linkahead/connection/authentication/external_credentials_provider.py

@@ -23,13 +23,10 @@
 #
 """external_credentials_provider."""
 from __future__ import absolute_import, unicode_literals
-from abc import ABCMeta
+from abc import ABC
 import logging
 from .plain import PlainTextCredentialsProvider
 
-# meta class compatible with Python 2 *and* 3:
-ABC = ABCMeta(str('ABC'), (object, ), {str('__slots__'): ()})
-
 
 class ExternalCredentialsProvider(PlainTextCredentialsProvider, ABC):
     """ExternalCredentialsProvider.