ci: pin python to 3.9 for test-cython job

at least until scipy is not available for 3.10, or else it fails with

https://github.com/fonttools/fonttools/runs/3997161938?check_suite_focus=true#step:5:23

.github/workflows/test.yml

@@ -70,7 +70,7 @@ jobs:
     - name: Set up Python 3.x
       uses: actions/setup-python@v2
       with:
-        python-version: "3.x"
+        python-version: 3.9
     - name: Install packages
       run: pip install tox
     - name: Run Tox