misc!: Drop support for Python 3.6 (#21)

Currently the CI for this repo won't run, even for a PR that aims to deprecate the repo. This pull request updates the version of Open Controls used and drops support for Python 3.6 (which current Open Controls doesn't support), so that the CI can run again.

Changes proposed in this pull request:
- Stop supporting Python 3.6.
- Remove unit tests for Python 3.6.
- Update version of Open Controls used.
- Fix imports for newer version of Open Controls.

BREAKING CHANGE: Drop support for Python 3.6

Author: leoadec

.github/workflows/on-push.yml

@@ -44,7 +44,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: [3.6, 3.7, 3.8]
+        python: [3.7, 3.8]
     steps:
     - uses: actions/checkout@v1
     - name: Install Python dependencies