Fix annotations on __exit__ and __aexit__ (#274)

* Fix annotations on `__exit__` and `__aexit__`

All parameters should be Optional, because they're None when no exception is thrown.

* changelog

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: JelleZijlstra

CHANGES/274.misc

@@ -0,0 +1 @@
+Improve type annotations for `__exit__` and `__aexit__` methods.

async_timeout/__init__.py

@@ -109,9 +109,9 @@ def __enter__(self) -> "Timeout":
 
     def __exit__(
         self,
-        exc_type: Type[BaseException],
-        exc_val: BaseException,
-        exc_tb: TracebackType,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
     ) -> Optional[bool]:
         self._do_exit(exc_type)
         return None
@@ -122,9 +122,9 @@ async def __aenter__(self) -> "Timeout":
 
     async def __aexit__(
         self,
-        exc_type: Type[BaseException],
-        exc_val: BaseException,
-        exc_tb: TracebackType,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
     ) -> Optional[bool]:
         self._do_exit(exc_type)
         return None
@@ -206,7 +206,7 @@ def _do_enter(self) -> None:
         self._state = _State.ENTER
         self._reschedule()
 
-    def _do_exit(self, exc_type: Type[BaseException]) -> None:
+    def _do_exit(self, exc_type: Optional[Type[BaseException]]) -> None:
         if exc_type is asyncio.CancelledError and self._state == _State.TIMEOUT:
             self._timeout_handler = None
             raise asyncio.TimeoutError