Remove subclass polyfill (#1156)

The subclass polyfill was only needed for Python 2.7-3.5

Python 3.6 introduced the __init_subclass__, so since Graphene now requires Python 3.6+, this is no longer needed.

https://www.python.org/dev/peps/pep-0487/

Author: syrusakbary

graphene/pyutils/init_subclass.py

@@ -1,10 +1,9 @@
 from inspect import isclass
 
-from ..pyutils.init_subclass import InitSubclassMeta
 from .props import props
 
 
-class SubclassWithMeta_Meta(InitSubclassMeta):
+class SubclassWithMeta_Meta(type):
     _meta = None
 
     def __str__(cls):