Enable --warn-redundant-casts option in mypy (#14671)

* Enable `--warn-redundant-casts` option in mypy

Doesn't do much but helps me sleep better at night.

* Changelog

* Fix name of the ignore

* Fix one more missed cast

Not sure why I didn't see this one locally, maybe I needed a poetry update

* Remove old comment

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

synapse/storage/database.py

@@ -667,7 +667,8 @@ class DatabasePool:
                 )
         # also check variables referenced in func's closure
         if inspect.isfunction(func):
-            f = cast(types.FunctionType, func)
+            # Keep the cast for now---it helps PyCharm to understand what `func` is.
+            f = cast(types.FunctionType, func)  # type: ignore[redundant-cast]
             if f.__closure__:
                 for i, cell in enumerate(f.__closure__):
                     if inspect.isgenerator(cell.cell_contents):