Add note about py 3.6 and lock tox env in workflow

Author: mikemanger

.github/workflows/tests.yml

@@ -9,6 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
+        # Python 3.6 is not supported by ubuntu-latest
         python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
 
     steps:
@@ -20,4 +21,4 @@ jobs:
     - name: Install dependencies
       run: pip install tox tox-gh
     - name: Test with tox
-      run: tox
+      run: tox -e ${{ matrix.python-version }}