Don't use metaclasses when class decorators can do the job.

Thanks to Nick Coghlan for pointing out that I'd forgotten about class
decorators.

Lib/email/policy.py

@@ -2,7 +2,7 @@
 code that adds all the email6 features.
 """
 
-from email._policybase import Policy, Compat32, compat32
+from email._policybase import Policy, Compat32, compat32, _extend_docstrings
 from email.utils import _has_surrogates
 from email.headerregistry import HeaderRegistry as HeaderRegistry
 
@@ -17,6 +17,7 @@ __all__ = [
     'HTTP',
     ]
 
+@_extend_docstrings
 class EmailPolicy(Policy):
 
     """+