Pypy in github actions

.github/workflows/tests.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, pypy2, pypy3]
 
     steps:
       - uses: actions/checkout@v2