Add versioning to deps in tox.ini, or remove -rc from options for isort check in tox.ini

[sudhir-b]

Bug Report

With isort v5.0.0 released recently, tox -e lint is failing, because the -rc option is no longer necessary as of v5.0.0 and is not recognised as a CLI option any more for isort.

We could avoid this by specifying versions of dependencies in the tox.ini file, or by removing the -rc option where it is specified.

Removal of the option is documented here: https://github.com/timothycrosley/isort/blob/develop/docs/upgrade_guides/5.0.0.md#--recursive-or--rc