Upgrade PySide to a more recent version.

[bcipriano]

Describe the process
Newer versions of Python 3 / pip on Windows no longer provide the version of PySide2 that we're using:

> pip3 install PySide2==5.11.2
ERROR: Could not find a version that satisfies the requirement PySide2==5.11.2 (from versions: 5.14.0, 5.14.1, 5.14.2, 5.14.2.1, 5.14.2.2, 5.14.2.3, 5.15.0)
ERROR: No matching distribution found for PySide2==5.11.2

We should upgrade to a more recent version to avoid affecting more Windows users.

[bcipriano]

Also just hit this issue running Python 3.8.5 on macOS:

Traceback (most recent call last):
  File "/Users/cipriano/homebrew/Cellar/python@3.8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Users/cipriano/homebrew/Cellar/python@3.8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/cipriano/opencue/cuegui/cuegui/__main__.py", line 24, in <module>
    import cuegui.Main
  File "/Users/cipriano/opencue/cuegui/cuegui/Main.py", line 33, in <module>
    import cuegui.MainWindow
  File "/Users/cipriano/opencue/cuegui/cuegui/MainWindow.py", line 45, in <module>
    class MainWindow(QtWidgets.QMainWindow):
TypeError: 'Shiboken.ObjectType' object is not iterable