pin mypy to a known good version (#7269)

Author: keewis

.github/workflows/ci-additional.yaml

@@ -119,7 +119,7 @@ jobs:
           python xarray/util/print_versions.py
       - name: Install mypy
         run: |
-          python -m pip install mypy
+          python -m pip install 'mypy<0.990'
 
       - name: Run mypy
         run: |