GitHub actions: test on Python 3.10 as well

Author: bfontaine

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.6, 3.7, 3.8, 3.9]
+        python-version: ['2.7', '3.6', '3.7', '3.8', '3.9', '3.10']
 
     steps:
     - uses: actions/checkout@v2