[PyQt] PyQt5.uic.loadUiType() ignores <customwidgets> for main widget class

[Kyle Altendorf]

It looks like presently PyQt5.uic.loadUiType() assumes that the .ui 
file's base widget's class is going to be in PyQt5.QtWidgets.  This is 
not the case for mine which often use a custom type specified in the 
<customwidgets> section.

Rather than:
     return (ui_globals[winfo["uiclass"]], getattr(QtWidgets, 
winfo["baseclass"]))

This seems to work:
     ui_base = ui_globals.get(winfo["baseclass"])

     if ui_base is None:
         ui_base = getattr(QtWidgets, winfo["baseclass"])

     return (ui_globals[winfo["uiclass"]], ui_base)

Example code and output are available at:
     https://gist.github.com/altendky/325d0737a923e6f678617364e0f68f83

Output:
     sys.version: 3.7.4 (default, Aug  2 2019, 12:58:45)
     [GCC 6.3.0 20170516]
     sys.platform: linux
     QT_VERSION_STR: 5.13.0
     PYQT_VERSION_STR: 5.13.0
     SIP_VERSION_STR: 4.19.18
     Traceback (most recent call last):
       File "bad3.py", line 50, in <module>
         main()
       File "bad3.py", line 46, in main
         Ui, UiBase = PyQt5.uic.loadUiType(io.StringIO(ui_text))
       File 
"/home/altendky/st.src/venv/lib/python3.7/site-packages/PyQt5/uic/__init__.py", 
line 204, in loadUiType
         return (ui_globals[winfo["uiclass"]], getattr(QtWidgets, 
winfo["baseclass"]))
     AttributeError: module 'PyQt5.QtWidgets' has no attribute 'MyWidget'

I'll attach my files again here but last time they didn't seem to be 
liked.  If anyone has suggestions on what I should do differently when 
attaching I'm happy to hear them.  I'm using RoundCube webmail.

Cheers,
-kyle
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