Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_weakref.pyi

@@ -0,0 +1,34 @@
+import sys
+from typing import Any, Callable, Generic, Optional, TypeVar, overload
+
+if sys.version_info >= (3, 9):
+    from types import GenericAlias
+
+_C = TypeVar("_C", bound=Callable[..., Any])
+_T = TypeVar("_T")
+
+class CallableProxyType(object):  # "weakcallableproxy"
+    def __getattr__(self, attr: str) -> Any: ...
+
+class ProxyType(object):  # "weakproxy"
+    def __getattr__(self, attr: str) -> Any: ...
+
+class ReferenceType(Generic[_T]):
+    if sys.version_info >= (3, 4):
+        __callback__: Callable[[ReferenceType[_T]], Any]
+    def __init__(self, o: _T, callback: Optional[Callable[[ReferenceType[_T]], Any]] = ...) -> None: ...
+    def __call__(self) -> Optional[_T]: ...
+    def __hash__(self) -> int: ...
+    if sys.version_info >= (3, 9):
+        def __class_getitem__(cls, item: Any) -> GenericAlias: ...
+
+ref = ReferenceType
+
+def getweakrefcount(__object: Any) -> int: ...
+def getweakrefs(object: Any) -> int: ...
+@overload
+def proxy(object: _C, callback: Optional[Callable[[_C], Any]] = ...) -> CallableProxyType: ...
+
+# Return CallableProxyType if object is callable, ProxyType otherwise
+@overload
+def proxy(object: _T, callback: Optional[Callable[[_T], Any]] = ...) -> Any: ...