Use isort instead of flake8-import-order

[edsq]

flake8-import-order is in maintenance mode and hasn't been updated for several years.

isort is being actively developed and seems to enjoy some popularity. This is a little different from flake8-import-order, as by default it actually re-writes code to fit its style, similarly to black. It also doesn't come with a first-party flake8 plugin, so we are using flake8-isort.

The intended behavior is therefore just a little different than it was with only flake8-import-order. While we still check for isort style compliance as part of the flake8 linting step in CI (using flake8-isort), we also now have a pre-commit hook that will run on commited files, and a nox session for running on all relevant files at once. The isort style is also a little different than we were previously using with flake8-import-order, so many files have changed.

This pull request:

• Removes flake8-import-order and associated configuration
• Adds isort and the associated flake8-isort plugin
• Configures pre-commit to automatically run isort on committed files
• Adds a nox session for running isort
• Updates all imports to be compatible with the isort style