Fix #198 bug where str subtype constraint does not work

[seratch]

This pull request fixes a bug brought by #198 . The pull request #198 enabled having sequence value for subtype in constraits.

@app.event({"type": "message", "subtype": ["file_share"]})
def handle_events(event):
    pass

Due to its incomplete implementation, the following code no longer works.

@app.event({"type": "message", "subtype": "file_share"})
def handle_events(event):
    pass

This pull request fixes this issue and we will be releasing a patch version shortly.

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 #203 (71c87a3) into main (c18e0d8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #203   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files         159      159           
  Lines        4744     4746    +2     
=======================================
+ Hits         4346     4348    +2     
  Misses        398      398           

Impacted Files	Coverage Δ
slack_bolt/listener_matcher/builtins.py	93.59% <100.00%> (+0.06%)	⬆️

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