Revert "✔ CircleCI: Do not test on Python 3.6 anymore"

This reverts commit 6d022f3c9e.

.circleci/config.yml

@@ -29,4 +29,4 @@ workflows:
                 - screenshots
           matrix:
             parameters:
-              python-image: ["python:3.7", "python:3.8", "python:3.9", "python:3.10"]
+              python-image: ["python:3.6", "python:3.7", "python:3.8", "python:3.9", "python:3.10"]