Fix #312 Type hint errors with pytype 2021.4.26

[seratch]

This pull request resolves #312 by updating some internals of the library code.

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]

Codecov Report

Merging #313 (d7765a4) into main (39194ae) will decrease coverage by 0.01%.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##             main     #313      +/-   ##
==========================================
- Coverage   91.47%   91.45%   -0.02%     
==========================================
  Files         166      166              
  Lines        5266     5278      +12     
==========================================
+ Hits         4817     4827      +10     
- Misses        449      451       +2     

Impacted Files	Coverage Δ
slack_bolt/request/async_request.py	91.42% <75.00%> (-2.12%)	⬇️
slack_bolt/request/request.py	94.11% <87.50%> (-2.55%)	⬇️
slack_bolt/oauth/async_callback_options.py	100.00% <100.00%> (ø)
slack_bolt/oauth/async_oauth_settings.py	100.00% <100.00%> (ø)
slack_bolt/oauth/oauth_settings.py	100.00% <100.00%> (ø)

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