ci: add python 3.10 to test matrix

3.10 was released recently.

https://docs.python.org/3.10/whatsnew/3.10.html

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.7, 3.8, 3.9]
+        python-version: [2.7, 3.7, 3.8, 3.9, 3.10]
 
     steps:
     - uses: actions/checkout@v2