Fix Python version declarations for 3.10

Author: necaris

.github/workflows/tests.yaml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.7", "3.8", "3.9", "3.10"]
+        python-version: ['3.7', '3.8', '3.9', '3.10', 'pypy-3.7', 'pypy-3.8']
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
     - uses: actions/checkout@master