Also pin mypy in the merge workflow (#1729)

.github/workflows/merge-typing.yml

@@ -27,7 +27,7 @@ jobs:
     - name: Install typing dependencies
       run: |
         python -m pip install -U pip
-        python -m pip install -U mypy
+        python -m pip install mypy==0.971
         python -m pip install -U types-requests types-pkg_resources types-setuptools
     - name: Run typing check with mypy
       run: |