Dvg-devices

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* Support for Numpy 2.0
* Restore support for Python 3.6. Was removed by mistake. Version 1.4.0 and this
version do support Python 3.6 actually, but only in combination with PyQt5.
PySide2 will not work because the wrapper library `qtpy` still expects
`app.exec_()` instead of `app.exec()`. That got fixed in the `qtpy` version
for Python >= 3.7.

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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

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* Added method ``BaseDevice.SerialDevice.query_bytes()``
* Fixed type hints in ``BaseDevice.SerialDevice``

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* Added support for PyQt5, PyQt6, PySide2 and PySide6

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* Added method ``BaseDevice.SerialDevice.readline()``

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