Fix compatibility with Flask 2.3 and newer.
Replace the removed _app_ctx_stack internal API with flask.g for
storing OAuth2 credentials in the current application context.
--- ../oauth2client/contrib/flask_util.py	2026-10-16 17:19:11.620950317 +0300
+++ ../oauth2client/contrib/flask_util.py	2026-10-16 17:19:46.825607429 +0300
@@ -170,8 +170,8 @@
 
 try:
     from flask import Blueprint
-    from flask import _app_ctx_stack
     from flask import current_app
+    from flask import g
     from flask import redirect
     from flask import request
     from flask import session
@@ -434,7 +434,7 @@
     @property
     def credentials(self):
         """The credentials for the current user or None if unavailable."""
-        ctx = _app_ctx_stack.top
+        ctx = g
 
         if not hasattr(ctx, _CREDENTIALS_KEY):
             ctx.google_oauth2_credentials = self.storage.get()