Use Generator instead of Iterator for 3rd-party context managers (#12481)

stubs/pyflakes/pyflakes/checker.pyi

@@ -1,6 +1,6 @@
 import ast
 import sys
-from collections.abc import Callable, Iterable, Iterator
+from collections.abc import Callable, Generator, Iterable, Iterator
 from contextlib import contextmanager
 from re import Pattern
 from typing import Any, ClassVar, Literal, TypeVar, overload
@@ -203,7 +203,7 @@ class Checker:
     @property
     def scope(self) -> Scope: ...
     @contextmanager
-    def in_scope(self, cls: Callable[[], Scope]) -> Iterator[None]: ...
+    def in_scope(self, cls: Callable[[], Scope]) -> Generator[None]: ...
     def checkDeadScopes(self) -> None: ...
     def report(self, messageClass: Callable[_P, Message], *args: _P.args, **kwargs: _P.kwargs) -> None: ...
     def getParent(self, node: ast.AST) -> ast.AST: ...