Add isort

[gadomski]

Related Issue(s):

• For some of the typing fixes needed for alternative constructors shouldn't hardcode their return types #862, I'm going to be doing a fair bit of import changes. Having isort on board will help reduce the mental load during those changes.

Description:

Adds isort to pre-commit and requirements-test.txt. There are no functional changes, all diffs should be either adding isort or applying isort.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.