Various stubtest exceptions (#5227)

stdlib/threading.pyi

@@ -54,7 +54,7 @@ class Thread:
             target: Optional[Callable[..., Any]] = ...,
             name: Optional[str] = ...,
             args: Iterable[Any] = ...,
-            kwargs: Mapping[str, Any] = ...,
+            kwargs: Optional[Mapping[str, Any]] = ...,
             *,
             daemon: Optional[bool] = ...,
         ) -> None: ...
@@ -65,7 +65,7 @@ class Thread:
             target: Optional[Callable[..., Any]] = ...,
             name: Optional[Text] = ...,
             args: Iterable[Any] = ...,
-            kwargs: Mapping[Text, Any] = ...,
+            kwargs: Optional[Mapping[Text, Any]] = ...,
         ) -> None: ...
     def start(self) -> None: ...
     def run(self) -> None: ...
@@ -130,14 +130,15 @@ class Condition:
 
 class Semaphore:
     def __init__(self, value: int = ...) -> None: ...
-    def __enter__(self) -> bool: ...
     def __exit__(
         self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[TracebackType]
     ) -> Optional[bool]: ...
     if sys.version_info >= (3,):
-        def acquire(self, blocking: bool = ..., timeout: float = ...) -> bool: ...
+        def acquire(self, blocking: bool = ..., timeout: Optional[float] = ...) -> bool: ...
+        def __enter__(self, blocking: bool = ..., timeout: Optional[float] = ...) -> bool: ...
     else:
         def acquire(self, blocking: bool = ...) -> bool: ...
+        def __enter__(self, blocking: bool = ...) -> bool: ...
     if sys.version_info >= (3, 9):
         def release(self, n: int = ...) -> None: ...
     else: