threading: __exit__ only returns None (#8298)

Fixes #8297 

`threading.Lock.__exit__` is implemented in C and can only return None: 6cbb57f62d/Modules/_threadmodule.c (L200)
`threading.Condition.__exit__` returns whatever its lock's `__exit__` returns, but our type annotations indicate that the lock is always a `Lock` or an `_RLock`, and neither returns anything other than None.

stdlib/threading.pyi

@@ -106,7 +106,7 @@ class Lock:
     def __enter__(self) -> bool: ...
     def __exit__(
         self, exc_type: type[BaseException] | None, exc_val: BaseException | None, exc_tb: TracebackType | None
-    ) -> bool | None: ...
+    ) -> None: ...
     def acquire(self, blocking: bool = ..., timeout: float = ...) -> bool: ...
     def release(self) -> None: ...
     def locked(self) -> bool: ...
@@ -125,7 +125,7 @@ class Condition:
     def __enter__(self) -> bool: ...
     def __exit__(
         self, exc_type: type[BaseException] | None, exc_val: BaseException | None, exc_tb: TracebackType | None
-    ) -> bool | None: ...
+    ) -> None: ...
     def acquire(self, blocking: bool = ..., timeout: float = ...) -> bool: ...
     def release(self) -> None: ...
     def wait(self, timeout: float | None = ...) -> bool: ...