PR: Replace custom implementation with loadUiType from PySide6

[JaRoSchm]

This replaces the custom implementation of qtpy.uic.loadUiType with the one from PySide6. Fixes #439.

[dalthviz]

Thank you @JaRoSchm for working on the fix! Could you also as part of this PR try to remove the skip for the test related with loadUiType at

qtpy/qtpy/tests/test_uic.py

Lines 58 to 60 in b303b62

	@pytest.mark.skipif(
	PYSIDE2 or PYSIDE6,
	reason="PySide2uic not consistently installed across platforms/versions")

Other than that this LGTM 👍 Thank you again!

[JaRoSchm]

Okay, looks like the pyside6-uic command which is necessary for PySide6.QtUiTools.loadUiType (see https://doc.qt.io/qtforpython-6/PySide6/QtUiTools/loadUiType.html) is not contained in the conda-forge version of PySide6 6.4.3 for Linux and macOS. Locally I use PySide6 6.5.1 on Ubuntu also installed from conda-forge and here it is present. Later I will try to find out more systematically in which versions it is present. This is probably what is ment by "PySide2uic not consistently installed across platforms/versions". This could mean that we should still skip the test (and sadly no vitables for my setup right now without larger changes on their side).

[JaRoSchm]

For context there was some discussion about this in #339, #265, #262.

[dalthviz]

Thank you for checking the skip @JaRoSchm ! Could you update the test skip then to follow up your findings and update the reason string to take into account the need for pyside6-uic command?

If that ends being up too complicated or you end up needing help let me know and I will try to help checking more in detail the skip definition 👍

[JaRoSchm]

I asked in conda-forge/pyside2-feedstock#193 where pyside6-uic should be available and it turned out that it is only packaged for Linux and macOS in version >= 6.5. QtPy uses 6.4 for its tests.

I tried to include this into the skip condition but somehow this only fixed the Python 3.7 tests. I don't get why the condition isn't true under Python 3.11. Could you please help me there?

[dalthviz]

Thank you again @JaRoSchm for all the effort here! Left a suggestion for the tests which hopefully will make the checks green🤞🏼

[dalthviz]

Thank you @JaRoSchm !