Fix following error occuring with isort v5+:
ERROR: Found resources import while parsing, but FIRSTPARTY was not included in the `sections` setting of your config. Please add it before continuing
See https://pycqa.github.io/isort/#custom-sections-and-ordering for more info.
* Upgrade to isort 5
isort 5 now can also handle imports not at top, hence some new locations now get sorted automatically.
* Move isort config from .isort.cfg to pyproject.toml
* isort: remove exclusion of some source files
isort now only ignores auto-generated files. The other files had been excluded because old isort could not handle imports not placed on the top.
* isort: use skip_glob to ignore resources.py
This should fix issues with the `skip` parameter on Windows.
