lint-and-test: Upgrade python version in mypy job for 1.5 to 3.8.

Author: neiljp

.github/workflows/lint-and-test.yml

@@ -23,7 +23,7 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-python@v4
         with:
-          python-version: 3.7
+          python-version: 3.8
           cache: 'pip'
           cache-dependency-path: 'setup.py'
       - name: Install with type-checking tools, stubs & minimal test libraries