Add pypy as a possible version

Author: rhpvorderman

.github/workflows/ci.yml

@@ -57,10 +57,12 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - 3.6
-          - 3.7
-          - 3.8
-          - 3.9
+          - "3.6"
+          - "3.7"
+          - "3.8"
+          - "3.9"
+          - "pypy-3.6"
+          - "pypy-3.7"
         os: ["ubuntu-latest"]
         include:
           - os: "macos-latest"