maint(precommit): Apply isort

[Skylion007]

Description

• Applies the isort formatter to the repo and enables the pre-commit hook for it. It is a simple formatter which is compatible with black and ensure that all imports in the repo occur in a standard (alphabetical) order unless otherwise indicated. It also otherwise can format the imports and ensure standard, third party, and first party libraries all go in their standard order. The ordering is automatically applied and enforced with a pre-commit hook.

Suggested changelog entry:

* Format pybind11 with isort consistent ordering of imports
* Enable isort pre-commit hook

[rwgk]

Cool!

[henryiii]

I'm obviously a fan. :)

[Skylion007]

Normally, these are autodetected, but it's a bit trickier in this project since the compiled modules don't live in the root, so we have to include any top level modules it might have missed (in tests)

[henryiii]

Sorry about the confusion, multi_line_output is part of "black".