TST: Pin to tox<4

[pllim]

Description

This pull request is to pin to tox<4 (has breaking changes) and add RC testing workflow.

[github-actions]

👋 Thank you for your draft pull request! Do you know that you can use [ci skip] or [skip ci] in your commit messages to skip running continuous integration tests until you are ready?

[codecov]

Codecov Report

Base: 97.08% // Head: 97.08% // No change to project coverage 👍

Coverage data is based on head (c1dc047) compared to base (a5fa653).
Patch has no changes to coverable lines.

❗ Current head c1dc047 differs from pull request most recent head 4d68a03. Consider uploading reports for the commit 4d68a03 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #343   +/-   ##
=======================================
  Coverage   97.08%   97.08%           
=======================================
  Files          17       17           
  Lines        2024     2024           
=======================================
  Hits         1965     1965           
  Misses         59       59           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.