Add isort as a formatter

[arogl]

WIP - Add isort as a formatter

Follows on from #4033

Both #4033 and this request require discussions about usage of pre-commit as a tool versus github actions

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)