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(