Python 3.6 and 3.7 are no longer available ubuntu-latest

Python versions less than 3.10 are eod-of-life.  GitHub Actions `ubuntu-latest` no longer supports Python versions less than 3.8 so let’s drop them so that our CI tests are green.

Author: cclauss

.github/workflows/ci.yml

@@ -10,18 +10,20 @@ jobs:
   tox:
     runs-on: ubuntu-latest
     strategy:
-      max-parallel: 5
       matrix:
         python-version:
-        - 3.6
-        - 3.7
         - 3.8
         - 3.9
+        - 3.10
+        - 3.11
+        - 3.12
+        - 3.13
+        - 3.14
         - pypy3
 
     steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-python@v2
+    - uses: actions/checkout@v6
+    - uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install tox