Fix #459 Invalid type hints in App / AsyncApp

[seratch]

This pull request fixes #459. Refer to the issue for details.

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 #460 (db30f04) into main (568c168) will not change coverage.
The diff coverage is n/a.

❗ Current head db30f04 differs from pull request most recent head 59516e3. Consider uploading reports for the commit 59516e3 to get more accurate results

@@           Coverage Diff           @@
##             main     #460   +/-   ##
=======================================
  Coverage   91.41%   91.41%           
=======================================
  Files         167      167           
  Lines        5523     5523           
=======================================
  Hits         5049     5049           
  Misses        474      474           

Impacted Files	Coverage Δ
slack_bolt/app/app.py	87.64% <ø> (ø)
slack_bolt/app/async_app.py	94.16% <ø> (ø)

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