CI: Add python 3.10 to test matrix

Author: fviard

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9]
+        python-version: ['2.7', '3.5', '3.6', '3.7', '3.8', '3.9', '3.10-dev']
     env:
       cache-revision: 1
     steps: