Make GSSError use the built-in metaclass syntax

Cython supports using the Python 3 syntax for setting metaclasses
(even in Python 2), and is able to optimize when it is used.

Author: DirectXMan12

gssapi/raw/misc.pyx

@@ -2,8 +2,6 @@ GSSAPI="BASE"  # This ensures that a full module is generated by Cython
 
 import locale  # for decoding error messages
 
-import six
-
 from gssapi.raw.cython_types cimport *
 from gssapi.raw.cython_converters cimport c_create_oid_set
 from gssapi.raw.names cimport Name
@@ -218,8 +216,7 @@ class GSSErrorRegistry(type):
 
 # NB(directxman12): this needs to be here (and not in another file)
 #                   so that display_status can use it
-@six.add_metaclass(GSSErrorRegistry)
-class GSSError(Exception):
+class GSSError(Exception, metaclass=GSSErrorRegistry):
     """
     GSSAPI Error