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/

graphene/pyutils/init_subclass.py

@@ -1,23 +0,0 @@
-is_init_subclass_available = hasattr(object, "__init_subclass__")
-
-if not is_init_subclass_available:
-
-    class InitSubclassMeta(type):
-        """Metaclass that implements PEP 487 protocol"""
-
-        def __new__(cls, name, bases, ns, **kwargs):
-            __init_subclass__ = ns.pop("__init_subclass__", None)
-            if __init_subclass__:
-                __init_subclass__ = classmethod(__init_subclass__)
-                ns["__init_subclass__"] = __init_subclass__
-            return super(InitSubclassMeta, cls).__new__(cls, name, bases, ns, **kwargs)
-
-        def __init__(cls, name, bases, ns, **kwargs):
-            super(InitSubclassMeta, cls).__init__(name, bases, ns)
-            super_class = super(cls, cls)
-            if hasattr(super_class, "__init_subclass__"):
-                super_class.__init_subclass__.__func__(cls, **kwargs)
-
-
-else:
-    InitSubclassMeta = type  # type: ignore

graphene/utils/subclass_with_meta.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):