Set worker thread names as str

[jarus]

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

None

❓ What is the current behavior? (You can also link to an open issue here)
Since the refactoring 7af66b0 the Cheroot worker threads are named like "('CP Server Thread-9',)".

❓ What is the new behavior (if this is a feature change)?
The worker threads are again correctly named "CP Server Thread-$i".

📋 Other information:
A leftover comma transformed the string to a tuple.

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• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
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This change is 

[the-allanc]

Any chance we can get this merged in?

[webknjaz]

Any chance we can get this merged in?

Sorry, I haven't been really watching FOSS those days but I'll merge this soon.

[webknjaz]

@jarus @pR0Ps thanks both of you for preparing the patches!