Remove usage of QVersionNumber because it doesn't work as expected in PySide6

[psobolewskiPhD]

This PR closes #567
If using pyside6 backend, QVersionNumber.fromString('5.15.2') returns a PySide6.QtCore.QVersionNumber and as a result it is not subscriptable, so launching the console results in a TypeError.
Because the code refers to Qt5 versions, I've added a check that the backend isn't Qt6 prior to the version checking.
On my mac this console now works in pyside6 6.4.2, though there is a warning:

python3.10/site-packages/jupyter_client/threaded.py:73: RuntimeWarning: ZMQStream only supports the base zmq.Socket class.

                Use zmq.Socket(shadow=other_socket)
                or `ctx.socket(zmq.SUB, socket_class=zmq.Socket)`
                to create a base zmq.Socket object,
                no matter what other kind of socket your Context creates.
                
  self.stream = zmqstream.ZMQStream(self.socket, self.ioloop)

I think that is unrelated to this PR and probably requires another fix.

[psobolewskiPhD]

Hmm, this PR shouldn't affect anything in the linux tests.
So I think the failed linux tests are likely because the ubuntu runners are now 22.04
actions/runner-images#5490
I guess something needs to be changed in the system apt install part of the workflow?

[akirchhoff-modular]

As one data point, I was encountering this issue as well, and this PR fixes it. The change looks straightforward and makes sense. I hope this can be merged soon.

[ccordoba12]

Thanks @psobolewskiPhD for your contribution!

[ccordoba12]

Besides my small suggestion below, please add an entry for packaging in setup.py because this is a new dependency. Specifically, you need to add it to this list:

qtconsole/setup.py

Lines 68 to 77 in 55a8fb0

	install_requires = [
	'traitlets!=5.2.1,!=5.2.2',
	'ipython_genutils',
	'jupyter_core',
	'jupyter_client>=4.1',
	'pygments',
	'ipykernel>=4.1', # not a real dependency, but require the reference kernel
	'qtpy>=2.0.1',
	'pyzmq>=17.1'
	],

[ccordoba12]

Looks good to me now, thanks @psobolewskiPhD!