Hardcoded -rc argument for isort is depracated

[abey79]

This action hardcodes -rc as argument to isort, but this triggers a deprecation warning:

 $ isort -rc -c --diff tests
/Users/hhip/src/vsketch/venv/lib/python3.8/site-packages/isort/main.py:771: UserWarning: 

The following deprecated CLI flags were used: -rc!
Please see the 5.0.0 upgrade guide:
        https://timothycrosley.github.io/isort/docs/upgrade_guides/5.0.0/

I suggest removing it altogether, it's easy to re-add via extra-isort-options if needed.

[ricardochaves]

I created a release of Isort 5 v1.2.0. But Pylint is not working with isort 5. So a updated the README.md with this limitation and Ill follow the Pylint problem, when it's fixed I update the repo.

[ricardochaves]

Pylint now supports isort 5. I created a release v1.3.0 with isort 5, pylint 2.6.0 and I removed -rc from the code.