Dev-env

* twine 3.4.2 -> 3.7.1
* pandoc 2.9.1.1 -> 2.16.2
* nvm 0.33.9 -> 0.39.1
* base docker image Ubuntu 18.04 -> 20.04
* Python 3.7 -> 3.9

Removed

* Nothing

Added

* add CircleCI docker executor [authenticated pull](https://circleci.com/docs/2.0/private-images/)
* per [this](https://discuss.circleci.com/t/old-linux-machine-image-remote-docker-deprecation/37572) article, added
explicit version to setup_remote_docker in CircleCI pipeline
* add docker image vulnerability scanning in CircleCI pipeline
* add LICENSE in repo's root directory

Changed

* updated dev env simonsdave/bionic-dev-env:latest -> simonsdave/focal-dev-env:latest
* start using run-codecov-uploader.sh

Removed

* Nothing

Added

* add [CodeFactor](https://www.codefactor.io/) badge to main README.md

Changed

* dev-env v0.6.7 -> v0.6.11

Removed

* Nothing

Added

* Nothing

Changed

* pip3 install -> python3.7 -m pip install
* simplification of docker builds

Removed

* Nothing

Added

* add [codecov](https://codecov.io/gh/simonsdave/dev-env-testing)

Changed

* dev_env v0.6.5 -> latest

Removed

* Nothing

Added

* add run-jsonlint.sh to CircleCI pipeline

Changed

* dev_env v0.6.1 -> v0.6.5

Removed

* Nothing