Add Python 3.14 to the testing

[cclauss]

Overview: What does this pull request change?

Add Python 3.14 to the testing

Motivation and Explanation: Why and how do your changes improve the library?

https://www.python.org/downloads/release/python-3140/

Blocked by:

• Pin version of pyav to avoid breaking changes in their API #4037
• PyAV is blocking the upgrade to Python 3.14 #4494

-    "av>=9.0.0,<14.0.0",
+    "av>=9.0.0,<14.0.0  ; python_full_version < '3.14'",
+    "av>=16.0.1 ; python_full_version >= '3.14'",

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested