use new pypy-3.6 specifier

This should fix the pypy3 testing error on github actions

.github/workflows/testing.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9, pypy-3.6]
 
     steps:
     - uses: actions/checkout@v2