Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/abc.pyi

@@ -0,0 +1,20 @@
+from typing import Any, Callable, Type, TypeVar
+
+_T = TypeVar("_T")
+_FuncT = TypeVar("_FuncT", bound=Callable[..., Any])
+
+# These definitions have special processing in mypy
+class ABCMeta(type):
+    def register(cls: ABCMeta, subclass: Type[_T]) -> Type[_T]: ...
+
+def abstractmethod(callable: _FuncT) -> _FuncT: ...
+
+class abstractproperty(property): ...
+
+# These two are deprecated and not supported by mypy
+def abstractstaticmethod(callable: _FuncT) -> _FuncT: ...
+def abstractclassmethod(callable: _FuncT) -> _FuncT: ...
+
+class ABC(metaclass=ABCMeta): ...
+
+def get_cache_token() -> object: ...