Test with newer, stable Python 3.10

I forgot to bump this version after 3.10 was released.

Author: jstasiak

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10.0-alpha.5, pypy3]
+        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", pypy3]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}