Set worker thread names as str

[jarus]

❓ What kind of change does this PR introduce?

• [X] 🐞 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.

📋 Contribution checklist:

(If you're a first-timer, check out this guide on making great pull requests)

• [x] I wrote descriptive pull request text above
• [x] I think the code is well written
• [x] I wrote good commit messages
• [x] I have squashed related commits together after the changes have been approved
• [ ] Unit tests for the changes exist
• [ ] Integration tests for the changes exist (if applicable)
• [x] I used the same coding conventions as the rest of the project
• [x] The new code doesn't generate linter offenses
• [ ] Documentation reflects the changes
• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

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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!