fix: use the real application object for Flask

[nblock]

According to Flask's documentation one needs to use the real application object instead of current_app [1]:

Passing Proxies as Senders

Never pass current_app as sender to a signal. Use
current_app._get_current_object() instead. The reason for this is that
current_app is a proxy and not the real application object.

[1] https://flask.palletsprojects.com/en/stable/signals/ (towards the end)

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What kind of change does this PR introduce?

A bugfix to reliably receive signals. It makes the example in https://docs.authlib.org/en/latest/client/flask.html#auto-update-token-via-signal work.

See #245

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover
• If this PR is about a new feature, or a behavior change, you have updated the documentation accordingly.

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