fix: remove appcontext which is removed in flask

[jb-leger]

I had the following bug:

File "/tmp/wex/lib/python3.11/site-packages/flask_cas/__init__.py", line 14, in <module>
 from flask import _request_ctx_stack as stack
ImportError: cannot import name '_request_ctx_stack' from 'flask' (/tmp/wex/lib/python3.11/site-packages/flask/__init__.py)

After some investigations: _app_ctx_stack and _request_ctx_stack were removed in flask. See this PR on flask.

In this extension, contexts are not used, these line are here as a copy of the flask extension dev doc. These line didn't have side effect, to the removal of this line is sufficiant to solve the bug.

[tg2648]

Hello @MasterRoshan, could you please take a look at this PR when you get a chance? It would be great to have flask-cas working with Flask 3.0. Thank you!