Update supported Python versions

Sync supported Python versions in CI with main Pelican repo (drop 3.6,
add 3.10 and 3.11).

Author: mirekdlugosz

.github/workflows/main.yml

@@ -12,7 +12,7 @@ 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", "3.10"]
 
     steps:
       - uses: actions/checkout@v2