Remove tests for python 3.6 from CI

Python 3.6 reached EOL 5 years ago and stopped receiving security updates last week. Dropping it from CI to speed up PR checks. See https://endoflife.date/python

Author: sambhav

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-latest, ubuntu-latest, windows-latest]
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v2