Remove nightly

It doesn't work with PyPy

Author: claudiosanches

.github/workflows/ci.yml

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