Improve ABCMeta.register signature (#962)

* Improve ABCMeta.register signature

* Fix ABCMeta.register return type for Python 3.3+

stdlib/3/abc.pyi

@@ -1,10 +1,15 @@
-from typing import Any
+from typing import Any, Type, TypeVar
 import sys
 # Stubs for abc.
 
+_T = TypeVar('_T')
+
 # Thesee definitions have special processing in type checker.
 class ABCMeta(type):
-    def register(cls: "ABCMeta", subclass: Any) -> None: ...
+    if sys.version_info >= (3, 3):
+        def register(cls: "ABCMeta", subclass: Type[_T]) -> Type[_T]: ...
+    else:
+        def register(cls: "ABCMeta", subclass: Type[Any]) -> None: ...
 abstractmethod = object()
 abstractproperty = object()