Make sure selected QtAPI actually provides QT support

[cv3d]

Maybe related issues: enthought/mayavi#448, enthought/mayavi#460, enthought/mayavi#483

PyFace might select PyQt5 as the default API if it is installed even if Qt5 itself is missing.
This PR fix such an issue by ensuring that at least QtCore can be imported from the being-selected API, otherwise try the next one.

[codecov-io]

Codecov Report

Merging #344 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #344   +/-   ##
=======================================
  Coverage   34.42%   34.42%           
=======================================
  Files         457      457           
  Lines       25432    25432           
  Branches     3797     3797           
=======================================
  Hits         8754     8754           
  Misses      16308    16308           
  Partials      370      370

Impacted Files	Coverage Δ
pyface/qt/__init__.py	47.5% <0%> (ø)	⬆️

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[corranwebster]

LGTM

Thanks for the contribution!

[cv3d]

@corranwebster You are welcome!
BTW: The order here is quite strange! pyqt should be considered before pyqt5, as per the current strategy to prefer Qt4 over 5.
Shall I PR on that as well?

[corranwebster]

I think that order is OK - it doesn't need to match the order in the files like QtCore.py, as they aren't expressing any sort of order/preference. In general, we prefer PySide over PyQt, and more recent over older, except that PySide2 is currently experimental for ETS (eg. currently tests are failing on windows).

[cv3d]

I see! I thought the warning Warning: The default toolkit if none is supplied is qt4. in the README implies that Qt4 is preferred..

[corranwebster]

Ah! No, that refers to the ETS_TOOLKIT not the PYFACE_API, which for historical reasons is called qt4 even if the PYFACE_API is pyqt5. We're planning to change that to qt in the future (and you can do that right now for some ETS projects), but that requires care since there is so much infrastructure we have that assumes ETS_TOOLKIT is qt4.

For the record, the priorities for qt vs wx are here: https://github.com/enthought/pyface/blob/master/pyface/base_toolkit.py#L94