Upgrade black (code formatter) to the latest version

[seratch]

This pull request upgrades black version to the latest (from 19.10b0 to 20.8b1) and applies the formatter to all code in this project.

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.

[codecov-io]

Codecov Report

Merging #167 (009e61c) into main (8efa2ba) will increase coverage by 0.00%.
The diff coverage is 89.18%.

@@           Coverage Diff           @@
##             main     #167   +/-   ##
=======================================
  Coverage   91.76%   91.77%           
=======================================
  Files         149      149           
  Lines        4446     4448    +2     
=======================================
+ Hits         4080     4082    +2     
  Misses        366      366           

Impacted Files	Coverage Δ
slack_bolt/adapter/aiohttp/__init__.py	42.85% <ø> (ø)
slack_bolt/adapter/aws_lambda/handler.py	92.42% <ø> (ø)
slack_bolt/adapter/pyramid/handler.py	89.18% <ø> (ø)
slack_bolt/adapter/sanic/async_handler.py	85.00% <0.00%> (ø)
slack_bolt/adapter/starlette/async_handler.py	87.09% <0.00%> (ø)
slack_bolt/adapter/starlette/handler.py	86.66% <0.00%> (ø)
slack_bolt/app/async_server.py	48.83% <ø> (ø)
slack_bolt/authorization/authorize.py	84.44% <ø> (ø)
slack_bolt/context/say/async_say.py	96.00% <ø> (ø)
slack_bolt/context/say/say.py	96.00% <ø> (ø)
... and 46 more

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