Require setuptools, cheroot/__init__.py imports pkg_resources

[hrnciar]

❓ 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 #)

❓ What is the current behavior? (You can also link to an open issue here)
pkg_resources needs setuptools as a runtime dependency. Since it wasn't specified so far, there was a possibility of ModuleNotFoundError: No module named 'pkg_resources' in case of setuptools not being present. There is a way how to install packages without setuptools or setuptools can be uninstalled after cheroot package was installed. My PR prevents this error.

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Checklist:

• I think the code is well written
• I wrote good commit messages
• 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)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• 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 

[webknjaz]

It's an optional dependency. This is why that import is wrapped with try/except. OTOH nowadays it's probably better to rely on importlib_metadata.