Add missing 3.15 classifier; release 1.20 started supporting 3.15 but didn't add the classifier.

Author: jamadden

setup.py

@@ -39,6 +39,7 @@ def _read(fname):
         'Programming Language :: Python :: 3.12',
         'Programming Language :: Python :: 3.13',
         'Programming Language :: Python :: 3.14',
+        'Programming Language :: Python :: 3.15',
         'Programming Language :: Python :: Implementation :: CPython',
         'Programming Language :: Python :: Implementation :: PyPy',
         'Framework :: Zope3',