Backport many Self-related changes to the Python-2 stdlib (#7128)

stdlib/@python2/sqlite3/dbapi2.pyi

@@ -1,3 +1,4 @@
+from _typeshed import Self
 from datetime import date, datetime, time
 from typing import Any, Callable, Generator, Iterable, Iterator, Protocol, Text, TypeVar
 
@@ -126,7 +127,7 @@ class Connection(object):
     def enable_load_extension(self, enabled: bool) -> None: ...
     def load_extension(self, path: str) -> None: ...
     def __call__(self, *args: Any, **kwargs: Any) -> Any: ...
-    def __enter__(self) -> Connection: ...
+    def __enter__(self: Self) -> Self: ...
     def __exit__(self, t: type | None, exc: BaseException | None, tb: Any | None) -> None: ...
 
 class Cursor(Iterator[Any]):