tests: silence a Python 3.14 specific warning; Mark Python 3.14 as being officially supported.

[mr-c]

Fixes: #767

For the Debian package of cheroot we need Python 3.14 support, and I noticed modifying and adapting the Python3.13 specific warning for Python 3.14 was enough to get the tests to pass with Python 3.14.

❓ 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 #)

Resolves #767

📋 Other information:

📋 Contribution checklist:

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• I wrote descriptive pull request text above
• 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
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  and description in grammatically correct, complete sentences

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.19%. Comparing base (662cd9d) to head (372d3e0).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #810      +/-   ##
==========================================
- Coverage   78.38%   78.19%   -0.19%     
==========================================
  Files          41       41              
  Lines        4788     4788              
  Branches      547      547              
==========================================
- Hits         3753     3744       -9     
- Misses        896      905       +9     
  Partials      139      139              

[mr-c]

• cheroot/test/test_ssl.py::test_tls_client_auth is flaky with Python 3.14, so I copied the existing filter warning
  

  cheroot/pytest.ini

  Line 63 in 662cd9d

  ignore:Exception ignored in. <function IOBase.__del__:pytest.PytestUnraisableExceptionWarning

  
  and adapted it to the Python 3.14 syntax
• I don't know why the doc builds are failing; perhaps that is unrelated?

[mr-c]

Doc failures are fixed in #811 ; I can include those here, if desired