Fix issues #36 and #38

[hokanosekai]

Hello, I have corrected a few problems related to the new version of flask.

I updated the issues #36 and #38 related to deprecated methods of flask such as the _request_ctx_stack and before_first_request.

For the _request_ctx_stack and _app_ctx_stack, i replaced it by removing old imports and import current_app from flask and update the get_app method in consequence by only returning the current_app if it's not None.

For the before_first_request, i remove the concerned line and add the @setupmethod decorator and a _check_setup_finished method to RBAC class.

Now the package is working under flask v3.0.2 and flask-login v0.6.3

This is my first PR on an open-source project, please tell me if i do things wrong 😄