Prevent matplotlib selecting the PyQt API instead of PySide2

[oliveroxtoby]

Running the FreeCAD daily build under Ubuntu 20.04, I have had the following error when the CfdOF workbench tries to create a plot:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/oliver/.FreeCAD/Mod/Plot/freecad/plot/Plot.py", line 110, in figure
    win = Plot(winTitle)
  File "/home/oliver/.FreeCAD/Mod/Plot/freecad/plot/Plot.py", line 423, in __init__
    self.canvas.setParent(self)
TypeError: arguments did not match any overloaded call:
  setParent(self, QWidget): argument 1 has unexpected type 'Plot'
  setParent(self, QWidget, Union[Qt.WindowFlags, Qt.WindowType]): argument 1 has unexpected type 'Plot'

This can be reproduced by entering the following in the python console.

from freecad.plot import Plot
Plot.figure("foo")

Further investigation revealed that on my system, when the modules are loaded, sys.modules contains 'PyQt.QtCore', causing matplotlib to select the PyQt5 API rather than PySide2 as it should. I don't know the reasons for this, but the attached commit forces matplotlib to select PySide2 by temporarily unloading the PyQt5.QtCore module if it is found.

[oliveroxtoby]

Thanks! :-)