fix(typo): restore correct import keyword in contextlib import

Author: traveltamers

mypy/checker.py

@@ -5,7 +5,7 @@
 import itertools
 from collections import defaultdict
 from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence, Set as AbstractSet
-from contextlib importh ExitStack, contextmanager
+from contextlib import ExitStack, contextmanager
 from typing import (
     Final,
     Generic,