Remove unsupported Pythons from CI

Both of 3.6 and 3.7 are deprecated, and the former is no longer
available in GH Actions.

Author: WhyNotHugo

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9"]
+        python-version: ["3.8", "3.9"]
 
     steps:
       - uses: "actions/checkout@v2"