add pypy Python versions 3.7, 3.8, and 3.9 to GitHub CI testing

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.x"]
+        python-version: ["3.7", "pypy-3.7", "3.8", "pypy-3.8", "3.9", "pypy-3.9", "3.10", "3.11", "3.x"]
 
     steps:
       - uses: actions/checkout@v3