Update workflow: remove unsupported Python 3.6 and fix matrix for ubuntu-latest

Author: tybruno

.github/workflows/pytest-workflow.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: ["3.6.7", "3.7", "3.8", "3.9", "3.10", "3.11"]
     steps:
       - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}