Add CI jobs that test on Python 3.11 and 3.12

Since some clients are using more recent versions than 3.10, it makes
sense to widen our CI matrix to mirror that.

Author: ldionne

.github/workflows/tox.yaml

@@ -10,7 +10,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.10", "3.11", "3.12"]
       fail-fast: false
 
     steps:

setup.py

@@ -75,6 +75,8 @@
         'Natural Language :: English',
         'Operating System :: OS Independent',
         'Programming Language :: Python :: 3.10',
+        'Programming Language :: Python :: 3.11',
+        'Programming Language :: Python :: 3.12',
         'Topic :: Software Development :: Quality Assurance',
         'Topic :: Software Development :: Testing',
     ],