Upgrade to PyQt6

[bicobus]

Changes within PyQt that affects us:

• All exec_() methods have been removed. ‼️
• The Qt module has been removed.
  -- which one ⁉️
• QtWidgets.qApp has been removed.
  -- Use QtWidgets.QApplication instead.
• pyrcc has been removed.

API changes within Qt6 that affects us:

• Classes moved from QtWidgets into QtGui
  • QAction and QActionGroup

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Currently, pipenv does not support PyQt6's wheel naming scheme. I'll fall back to python venv if pipenv doesn't have a new release soon(ish).

[bicobus]

QtCore.QEvent.type() is currently buggy in PyQt 6.0.0:

https://www.riverbankcomputing.com/pipermail/pyqt/2021-January/043498.html