Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/@python2/abc.pyi

@@ -0,0 +1,31 @@
+import _weakrefset
+from typing import Any, Callable, Dict, Set, Tuple, Type, TypeVar
+
+_FuncT = TypeVar("_FuncT", bound=Callable[..., Any])
+
+# NOTE: mypy has special processing for ABCMeta and abstractmethod.
+
+def abstractmethod(funcobj: _FuncT) -> _FuncT: ...
+
+class ABCMeta(type):
+    # TODO: FrozenSet
+    __abstractmethods__: Set[Any]
+    _abc_cache: _weakrefset.WeakSet[Any]
+    _abc_invalidation_counter: int
+    _abc_negative_cache: _weakrefset.WeakSet[Any]
+    _abc_negative_cache_version: int
+    _abc_registry: _weakrefset.WeakSet[Any]
+    def __init__(self, name: str, bases: Tuple[type, ...], namespace: Dict[Any, Any]) -> None: ...
+    def __instancecheck__(cls: ABCMeta, instance: Any) -> Any: ...
+    def __subclasscheck__(cls: ABCMeta, subclass: Any) -> Any: ...
+    def _dump_registry(cls: ABCMeta, *args: Any, **kwargs: Any) -> None: ...
+    def register(cls: ABCMeta, subclass: Type[Any]) -> None: ...
+
+# TODO: The real abc.abstractproperty inherits from "property".
+class abstractproperty(object):
+    def __new__(cls, func: Any) -> Any: ...
+    __isabstractmethod__: bool
+    doc: Any
+    fdel: Any
+    fget: Any
+    fset: Any