add shim for nullcontext

Author: ikonst

mypy/checkexpr.py

@@ -1,7 +1,7 @@
 """Expression type checker. This file is conceptually part of TypeChecker."""
-
+from mypy.nullcontext import nullcontext
 from mypy.ordered_dict import OrderedDict
-from contextlib import contextmanager, nullcontext
+from contextlib import contextmanager
 import itertools
 from typing import (
     Any, cast, Dict, Set, List, Tuple, Callable, Union, Optional, Sequence, Iterator

mypy/nullcontext.py

@@ -0,0 +1,10 @@
+import sys
+from contextlib import contextmanager
+from typing import Iterator
+
+if sys.version_info < (3, 7):
+    @contextmanager
+    def nullcontext() -> Iterator[None]:
+        yield
+else:
+    from contextlib import nullcontext as nullcontext, contextmanager  # noqa: F401