Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_weakrefset.pyi

@@ -0,0 +1,47 @@
+import sys
+from typing import Any, Generic, Iterable, Iterator, MutableSet, Optional, TypeVar, Union
+
+if sys.version_info >= (3, 9):
+    from types import GenericAlias
+
+_S = TypeVar("_S")
+_T = TypeVar("_T")
+_SelfT = TypeVar("_SelfT", bound=WeakSet[Any])
+
+class WeakSet(MutableSet[_T], Generic[_T]):
+    def __init__(self, data: Optional[Iterable[_T]] = ...) -> None: ...
+    def add(self, item: _T) -> None: ...
+    def clear(self) -> None: ...
+    def discard(self, item: _T) -> None: ...
+    def copy(self: _SelfT) -> _SelfT: ...
+    def pop(self) -> _T: ...
+    def remove(self, item: _T) -> None: ...
+    def update(self, other: Iterable[_T]) -> None: ...
+    def __contains__(self, item: object) -> bool: ...
+    def __len__(self) -> int: ...
+    def __iter__(self) -> Iterator[_T]: ...
+    def __ior__(self, other: Iterable[_S]) -> WeakSet[Union[_S, _T]]: ...
+    def difference(self: _SelfT, other: Iterable[_T]) -> _SelfT: ...
+    def __sub__(self: _SelfT, other: Iterable[_T]) -> _SelfT: ...
+    def difference_update(self: _SelfT, other: Iterable[_T]) -> None: ...
+    def __isub__(self: _SelfT, other: Iterable[_T]) -> _SelfT: ...
+    def intersection(self: _SelfT, other: Iterable[_T]) -> _SelfT: ...
+    def __and__(self: _SelfT, other: Iterable[_T]) -> _SelfT: ...
+    def intersection_update(self, other: Iterable[_T]) -> None: ...
+    def __iand__(self: _SelfT, other: Iterable[_T]) -> _SelfT: ...
+    def issubset(self, other: Iterable[_T]) -> bool: ...
+    def __le__(self, other: Iterable[_T]) -> bool: ...
+    def __lt__(self, other: Iterable[_T]) -> bool: ...
+    def issuperset(self, other: Iterable[_T]) -> bool: ...
+    def __ge__(self, other: Iterable[_T]) -> bool: ...
+    def __gt__(self, other: Iterable[_T]) -> bool: ...
+    def __eq__(self, other: object) -> bool: ...
+    def symmetric_difference(self, other: Iterable[_S]) -> WeakSet[Union[_S, _T]]: ...
+    def __xor__(self, other: Iterable[_S]) -> WeakSet[Union[_S, _T]]: ...
+    def symmetric_difference_update(self, other: Iterable[_S]) -> None: ...
+    def __ixor__(self, other: Iterable[_S]) -> WeakSet[Union[_S, _T]]: ...
+    def union(self, other: Iterable[_S]) -> WeakSet[Union[_S, _T]]: ...
+    def __or__(self, other: Iterable[_S]) -> WeakSet[Union[_S, _T]]: ...
+    def isdisjoint(self, other: Iterable[_T]) -> bool: ...
+    if sys.version_info >= (3, 9):
+        def __class_getitem__(cls, item: Any) -> GenericAlias: ...