Replace Flask-Sockets with aiohttp for testing

[WilliamBergamin]

This PR bring the logic from slack-sdk #1445 to bolt python

Flask-Sockets is deprecated and it won't receive any fixes in the future, so it seems reasonable to replace it with some up-to-date library. aiohttp has been chosen since it was already presented as an optional requirement.

Category (place an x in each of the [ ])

• [ ] slack_bolt.App and/or its core components
• [ ] slack_bolt.async_app.AsyncApp and/or its core components
• [ ] Adapters in slack_bolt.adapter
• [ ] Document pages under /docs
• [x] Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f150cfd) 91.76% compared to head (3a8f641) 91.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1012 +/- ## ======================================= Coverage 91.76% 91.76% ======================================= Files 181 181 Lines 6312 6312 ======================================= Hits 5792 5792 Misses 520 520 ```

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