update python versions

Author: matthewhanson

.github/workflows/continuous-integration.yml

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