Drop python 3.7, add python 3.12

Matching the Python support schedule:  https://devguide.python.org/versions/

Author: bgrant

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11']
+        python-version: ['3.8', '3.9', '3.10', '3.11', '3.12']
 
     steps:
     - uses: actions/checkout@v3