Add _py_abc stubs (#4129)

stdlib/3.7/_py_abc.pyi

@@ -0,0 +1,11 @@
+
+from typing import Type, TypeVar, Tuple, Any, Dict, NewType
+
+_T = TypeVar('_T')
+
+# TODO: Change the return into a NewType bound to int after pytype/#597
+def get_cache_token() -> object: ...
+
+class ABCMeta(type):
+    def __new__(mcls, __name: str, __bases: Tuple[Type[Any], ...], __namespace: Dict[str, Any]) -> ABCMeta: ...
+    def register(cls, subclass: Type[_T]) -> Type[_T]: ...