Require black and isort for contributions (#3329)

* Add explanation to CONTRIBUTNG.md
* Add sample pre-commit script
* Check for correctly formatted files in CI

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stdlib/2and3/difflib.pyi

@@ -57,10 +57,7 @@ class SequenceMatcher(Generic[_T]):
 # mypy thinks the signatures of the overloads overlap, but the types still work fine
 @overload
 def get_close_matches(  # type: ignore
-    word: AnyStr,
-    possibilities: Iterable[AnyStr],
-    n: int = ...,
-    cutoff: float = ...,
+    word: AnyStr, possibilities: Iterable[AnyStr], n: int = ..., cutoff: float = ...,
 ) -> List[AnyStr]: ...
 @overload
 def get_close_matches(

stdlib/3/asyncio/base_events.pyi

@@ -73,10 +73,7 @@ class BaseEventLoop(AbstractEventLoop, metaclass=ABCMeta):
     # run_in_executor is defined as a coroutine in AbstractEventLoop but returns a Future in concrete implementation.
     # Need to ignore mypy Return type error as a result.
     def run_in_executor(  # type: ignore
-        self,
-        executor: Any,
-        func: Callable[..., _T],
-        *args: Any,
+        self, executor: Any, func: Callable[..., _T], *args: Any,
     ) -> Future[_T]: ...
     def set_default_executor(self, executor: Any) -> None: ...
     # Network I/O methods returning Futures.