CI: pin mypy to version 1.8

Version 1.9 removes support for python 3.7, so we either need to pin the
version to 1.8 as long as we're support Python 3.7, or we need to drop
linting for 3.7

.github/workflows/lint.yml

@@ -46,7 +46,8 @@ jobs:
     - uses: actions/setup-python@v4
       with:
         python-version: '3.x'
-    - run: python -m pip install mypy coverage strictyaml types-PyYAML types-tqdm types-chevron
+    # Pin mypy to version 1.8, so we retain the ability to lint for Python 3.7
+    - run: python -m pip install "mypy==1.8" coverage strictyaml types-PyYAML types-tqdm types-chevron
     - run: python run_mypy.py --allver
       env:
         PYTHONUNBUFFERED: 1