build: add a free-threading classifier

Author: nedbat

setup.py

@@ -28,6 +28,7 @@
 Programming Language :: Python :: 3.13
 Programming Language :: Python :: 3.14
 Programming Language :: Python :: 3.15
+Programming Language :: Python :: Free Threading :: 3 - Stable
 Programming Language :: Python :: Implementation :: CPython
 Programming Language :: Python :: Implementation :: PyPy
 Topic :: Software Development :: Quality Assurance