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

[mr-c]

Works towards: #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 #)

#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
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@51b7d62). Learn more about missing BASE report.
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #810   +/-   ##
=======================================
  Coverage        ?   78.19%           
=======================================
  Files           ?       41           
  Lines           ?     4788           
  Branches        ?      547           
=======================================
  Hits            ?     3744           
  Misses          ?      905           
  Partials        ?      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

[webknjaz]

Fixes: #767

Not sure if we should be closing that issue with this. It's probably best to keep it open for the follow-up fixes since this PR only suppresses warnings that do need fixing at some point.