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Major clean-up and streamlining:
* Using `qtpy` library instead of my own Qt5/6 mechanism
* Changed all string formatting to f-strings
* Extended type hinting and checking
* Made demos uniform and passing `qdev` arguments to `MainWindow` now
* Individual source files now follow the PyPi package version
* Resolved nearly all Pylint / Pylance warnings
* Removed Python 3.6 support
New devices added:
* Xylem Hydrovar HVL - Variable speed pump controller
* Novanta IMS MDrive - Stepper motor controller