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:

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