Fix type hint errors detected by pytype 2021.7.19

[seratch]

This pull request fixes the CI build errors with the latest version of pytype.

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 #416 (2c9a40d) into main (7920f39) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 2c9a40d differs from pull request most recent head e21e0a5. Consider uploading reports for the commit e21e0a5 to get more accurate results

@@           Coverage Diff           @@
##             main     #416   +/-   ##
=======================================
  Coverage   91.36%   91.36%           
=======================================
  Files         167      167           
  Lines        5488     5491    +3     
=======================================
+ Hits         5014     5017    +3     
  Misses        474      474           

Impacted Files	Coverage Δ
...ack_bolt/adapter/aws_lambda/local_lambda_client.py	100.00% <ø> (ø)
slack_bolt/middleware/ssl_check/ssl_check.py	93.10% <100.00%> (+0.79%)	⬆️

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