[PyQt] Potential regression with PyQt5-5.7: locked widget layout

[Ghislain Vaillant]

Forwarding this downstream bug from the Pyzo IDE [1], which potentially
suffers from a regression from PyQt5.7 based on my initial observations.

The summary of the issue is the following:

The Pyzo IDE recently enabled usage of PyQt5 as a GUI backend using the 
popular QtPy shim library.

The IDE worked fine with the PyQt4 and PyQt5 backends until PyQt5-5.6
included. However, since PyQt5-5.7, the location of the widgets cannot
be changed. The widgets can be grabbed and moved around but they do not
stick to the target location on release of the mouse click and return
back to their original location.

To reproduce the problem in a distribution with Python3 >= 3.5 (I used
Debian testing)

```
pyvenv pyzo
source pyzo/bin/active
pip install -U pip setuptools wheel
pip install pyzo qtpy pyqt5==5.6
pyzo
```

This should work, widgets can be dragged, moved and relocated freely.

```
pip install pyqt5==5.7
pyzo
```

The widgets can be dragged and moved but their location cannot be
changed.

Any idea where the issue comes from? Feel free to respond to the bug
report directly if you prefer.

[1] https://github.com/pyzo/pyzo/issues/443

Best regards,
Ghis