Add isort to linters

[mattwthompson]

Mirrors #677 but with isort, keeping with the spirit of minimizing the scope of each PR

• Update linting action
• update setup.cfg with isort settings - a notable difference between most linters and black, which typically takes no settings
• Run isort
• Tag issue being addressed
• Add tests
• Update docstrings/documentation, if applicable
• Update changelog

[j-wags]

Looks great. Before merging, could you also update the PR template to explain the linting steps to submitters? Maybe just add it as a checkbox, and link to the appropriate instructions in the docs?

[mattwthompson]

Done - thanks @j-wags!