Bump python to 3.7 and 3.8

Author: aaronn

.github/workflows/runtests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: [3.6, 3.7]
+        python: [3.7, 3.8]
 
     steps:
       - uses: actions/checkout@v1