Dropped support for Python 3.6

Author: JohanMabille

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu, macos, windows]
-        python-version: [ '3.6', '3.7', '3.8', '3.9', 'pypy3' ]
+        python-version: [ '3.7', '3.8', '3.9' ]
         exclude:
         - os: windows
           python-version: pypy3