This PR updates PyQt5 and it's associated libraries to a newer version (from version 5.15.1 to 5.15.6) and adds the PyQt5-stubs package to the lint tox testenv, effectively allowing mypy's type checking to now work against (most?) PyQt5 objects. Prior to this change, mypy's "has no attribute" error was being suppressed globally to account for the missing stubs (which resulted in legitimate attribute errors going uncaught -- and in mypy's type checks, more generally, being less useful to this project).

As a consequence of this change -- or, more specifically, due to this Qt bug -- support for Debian 10/"Buster"/oldstable will be dropped (but may be reinstated if/when PyQt5 ships the newly-open-sourced Qt 5.15.3 LTS).