Add Python 3.10 to the supported versions

[seratch]

This pull request adds a CI build job with Python 3.10 and updates the package metadata.

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
• 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.

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

[seratch]

Need to upgrade pytest to 6.x to run tests in Python 3.10: https://github.com/pytest-dev/pytest/releases/tag/6.2.5

[codecov]

Codecov Report

Merging #497 (ab71d2d) into main (81d0e7a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #497   +/-   ##
=======================================
  Coverage   91.43%   91.43%           
=======================================
  Files         167      167           
  Lines        5533     5533           
=======================================
  Hits         5059     5059           
  Misses        474      474           

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