Add Python 3.10 to test matrix

Author: krassowski

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9, '3.10']
 
     steps:
     - uses: actions/checkout@v2