add pypy3 to test matrix

Author: bollwyvl

.github/workflows/main.yml

@@ -69,8 +69,10 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
-
+        python-version: [3.6, 3.7, 3.8, 3.9, pypy-3.7]
+        exclude:
+        - os: windows-latest
+          python-version: pypy-3.7
     env:
       OS: ${{ matrix.os }}