🔧 MAINTAIN: Pin PyPy Python version (#138)

Author: hukkinj1

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [pypy3, 3.6, 3.7, 3.8, 3.9]
+        python-version: [pypy-3.6, 3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2