Upgrade to Python 3.10.

Shapely doesn't have a Windows wheel on pypi yet, but we can get them
from https://www.lfd.uci.edu/~gohlke/pythonlibs/#shapely. This unblocks
(and because wheels are matched to python versions, *requires*)
upgrading to Python 3.10.

.github/workflows/build.yml

@@ -10,10 +10,10 @@ jobs:
         with:
           submodules: true
 
-      - name: Set up Python 3.9
+      - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: 3.9
+          python-version: "3.10"
 
       - name: Install environment
         run: |