python 3.6 isn't available on the latest ubuntu image

Author: martinthomson

.github/workflows/python.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.6', '3.7', '3.8', '3.9']
+        python-version: ['3.7', '3.8', '3.9']
 
     steps:
     - uses: actions/checkout@v2