fix workflow

Author: bollwyvl

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
         python-version: [ '3.6', '3.7', '3.8', '3.9', 'pypy3' ]
         exclude:
         - os: windows
-          PYTHON_VERSION: pypy3
+          python-version: pypy3
     steps:
     - name: Checkout
       uses: actions/checkout@v1