Delete python 2 branches from third-party stubs (#7741)

Since #7703, we no longer have third-party stubs that support Python 2, so code like `if sys.version_info >= (3, 0)` can be simplified.

stubs/requests/requests/cookies.pyi

@@ -1,12 +1,7 @@
-import sys
 from collections.abc import MutableMapping
+from http.cookiejar import CookieJar
 from typing import Any
 
-if sys.version_info >= (3, 0):
-    from http.cookiejar import CookieJar
-else:
-    from cookielib import CookieJar
-
 class MockRequest:
     type: Any
     def __init__(self, request) -> None: ...