setup.cfg: add Python 3.14 classifier

We've been running our test suite on Python 3.14 since November 2025,
see kaitai-io/ci_artifacts@6fe3566

The switch from Python 3.13 to Python 3.14 did not cause any changes in
test results, so as far as we know, Python 3.14 is fully supported.

Author: generalmimon

setup.cfg

@@ -25,6 +25,7 @@ classifiers =
     Programming Language :: Python :: 3.11
     Programming Language :: Python :: 3.12
     Programming Language :: Python :: 3.13
+    Programming Language :: Python :: 3.14
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy