fix: handle qmods like QT5 for pyside6 not 6.4

[psobolewskiPhD]

We're trying to Goldilocks a pyside6 version for napari tests.
Recently (napari/napari#6738) when testing with pyside6 6.3.1, we got this failure:
https://github.com/napari/napari/actions/runs/8303510051/job/22727974608?pr=6738#step:12:209

Traceback (most recent call last):
    File "/home/runner/work/napari/napari/napari/_qt/widgets/qt_keyboard_settings.py", line 685, in keyPressEvent
      kb = qkeysequence2modelkeybinding(event_keyseq)
    File "/home/runner/work/napari/napari/.tox/py39-linux-pyside6-cov/lib/python3.9/site-packages/app_model/backends/qt/_qkeymap.py", line 316, in qkeysequence2modelkeybinding
      parts = [SimpleKeyBinding.from_int(qkeycombo2modelkey(x)) for x in iter(key)]
    File "/home/runner/work/napari/napari/.tox/py39-linux-pyside6-cov/lib/python3.9/site-packages/app_model/backends/qt/_qkeymap.py", line 316, in <listcomp>
      parts = [SimpleKeyBinding.from_int(qkeycombo2modelkey(x)) for x in iter(key)]
    File "/home/runner/work/napari/napari/.tox/py39-linux-pyside6-cov/lib/python3.9/site-packages/app_model/backends/qt/_qkeymap.py", line 308, in qkeycombo2modelkey
      qmods = _get_qmods(key)
    File "/home/runner/work/napari/napari/.tox/py39-linux-pyside6-cov/lib/python3.9/site-packages/app_model/backends/qt/_qkeymap.py", line 85, in _get_qmods
      return key.keyboardModifiers()
  AttributeError: 'int' object has no attribute 'keyboardModifiers'

In this PR, for versions of pyside6 before 6.4, just use the QT5 handling.
I tested locally with pyside 6.3.1, and latest, as well as pyqt6 6.3.1 and latest.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (03f5e1a) to head (a9e7265).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #179   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           31        31           
  Lines         1785      1786    +1     
=========================================
+ Hits          1785      1786    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tlambert03]

works for me.

fwiw, i think the underlying cause here is the fact that the behavior of QKeySequence.__iter__ (called here) is changing. Another way to fix this would be, in qkeycombo2modelkey, to use:

if isinstance(key, int):
    key = QKeyCombination.fromCombined(key)

which is a generic (Qt6+) way to cast an integer to a QKeyCombination. but i'll add more qt version testing in a follow up PR and consider a change then. Thanks @psobolewskiPhD

[tlambert03]

@psobolewskiPhD, we're now testing everything from Qt 5.15; 6.2-6.6 #180