Add support for socket mode request body's for python 3.6+

[JordanGibson]

Category

• slack_bolt.App and/or its core components (slack_bolt.BoltRequest)
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

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.

This PR will resolve issue #552

Results after running all tests before changes:

Results after running all tests after changes:

[CLAassistant]

All committers have signed the CLA.

[seratch]

Thanks for taking the time to make this PR! but let us close this one in favor of #555