Invalid `isort` options in `setup.cfg`

I noticed isort is not working because `THIRD_PARTY` should be `THIRDPARTY` (https://pycqa.github.io/isort/docs/configuration/custom_sections_and_ordering.html).

Additionally I'd propose the settings:
```
profile = black
skip_gitignore = true
```
 
to enable black compatibility mode.

This touches quite a lot of files. Shall I submit a PR for that?