Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/multiprocessing/synchronize.pyi

@@ -0,0 +1,47 @@
+import sys
+import threading
+from multiprocessing.context import BaseContext
+from typing import Any, Callable, ContextManager, Optional, Union
+
+_LockLike = Union[Lock, RLock]
+
+class Barrier(threading.Barrier):
+    def __init__(
+        self, parties: int, action: Optional[Callable[..., Any]] = ..., timeout: Optional[float] = ..., *ctx: BaseContext
+    ) -> None: ...
+
+class BoundedSemaphore(Semaphore):
+    def __init__(self, value: int = ..., *, ctx: BaseContext) -> None: ...
+
+class Condition(ContextManager[bool]):
+    def __init__(self, lock: Optional[_LockLike] = ..., *, ctx: BaseContext) -> None: ...
+    if sys.version_info >= (3, 7):
+        def notify(self, n: int = ...) -> None: ...
+    else:
+        def notify(self) -> None: ...
+    def notify_all(self) -> None: ...
+    def wait(self, timeout: Optional[float] = ...) -> bool: ...
+    def wait_for(self, predicate: Callable[[], bool], timeout: Optional[float] = ...) -> bool: ...
+    def acquire(self, block: bool = ..., timeout: Optional[float] = ...) -> bool: ...
+    def release(self) -> None: ...
+
+class Event(ContextManager[bool]):
+    def __init__(self, lock: Optional[_LockLike] = ..., *, ctx: BaseContext) -> None: ...
+    def is_set(self) -> bool: ...
+    def set(self) -> None: ...
+    def clear(self) -> None: ...
+    def wait(self, timeout: Optional[float] = ...) -> bool: ...
+
+class Lock(SemLock):
+    def __init__(self, *, ctx: BaseContext) -> None: ...
+
+class RLock(SemLock):
+    def __init__(self, *, ctx: BaseContext) -> None: ...
+
+class Semaphore(SemLock):
+    def __init__(self, value: int = ..., *, ctx: BaseContext) -> None: ...
+
+# Not part of public API
+class SemLock(ContextManager[bool]):
+    def acquire(self, block: bool = ..., timeout: Optional[float] = ...) -> bool: ...
+    def release(self) -> None: ...