Backport PR
#8829: Qt5 fix
Fix for issue
#8757.
The problem was that IPython.external.qt was moved to qtconsole.qt, but IPython.external.qt_for_kernel was still trying to import IPython.external.qt.
Rather than copying the qt.py file back to external, this PR merges the logic back into qt_for_kernel.py. Deferring to qt.py was a bit silly because both qt.py and qt_for_kernel.py do basically the same thing but in slightly different ways. Moreover, the assumption of qt.py was that if the QT_API environment variable is set, it must be in an ETS environment and need to comply by the ETS import logic. This is not valid, as ipython sets the QT_API environment variable to communicate intent as to Qt version, in `IPython/lib/inputhook.py:385`
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