Ref #6463, #6428
* pin isort to 5.6.4
* update isort version mentioned in python_guidelines.rst
* make lint commands consistent between make lint and circleci
* fix isort errors on files outside of /wagtail
* speed up isort by ignoring additional directories and filetypes
* Added CI config to generate nightly builds from master
* Fix typo
* Don't write __init__.py directly
We need to import the original in the script
* Update wagtail.utils.version
This is based off a copy from Django master made just now and adds
support for 'dev' versions.
