Remove email_manager. Use IMAPClient instead.
- For IMAPClient, see https://imapclient.readthedocs.io/en/2.1.0/
- Since there's a better solution out there for automated email testing, I'm going to remove email testing functionality from SeleniumBase in favor of other tools that use IMAP. It didn't make much sense to bundle it with SeleniumBase since it never used any of the browser-related functionality.