* mysql_user: fixed encrypted option for MySQL 8.0 and test coverage
The purpose of this change was originally to expand test coverage to
unblock #76, but an issue was detected with the encrypted parameter on
MySQL 8.0 in the process of writing the tests. Additionally,
user_password_update_test.yml had been disabled at some point, so I
opted to replace it with two new files that will focus on the password
and plugin auth paths.
* Updated tests to cover a couple of missing branches
* Skip tests that rely on sha256_password if pymysql < 0.9
* Cover the case where pymysql isn't installed for plugin tests
* Added better plugin auth checking to tests and other minor changes
* Fixed version detection to explicitly handle MariaDB
* Removed unneeded import from previous change
* Remove whitespace that was introduced by change that was removed
* Added unit tests for missing coverage
