DO-NOT-MERGE: Use Type Alias instead of import in csv.pyi

I came across a mypy bug while working on python#6717; this PR is just so that I can get a good repro for a bug report to mypy.

Author: AlexWaygood

stdlib/csv.pyi

@@ -21,7 +21,7 @@ from collections.abc import Collection, Iterable, Iterator, Mapping, Sequence
 from typing import Any, Generic, Type, TypeVar, overload
 
 if sys.version_info >= (3, 8):
-    from builtins import dict as _DictReadMapping
+    _DictReadMapping = dict
 else:
     from collections import OrderedDict as _DictReadMapping