[PyQt] Wrong unicode text encoding in the embedded :qt/etc/qt.conf.

[Ales Erjavec]

Hi,

When PyQt5 (wheel) is installed on a path containing non-latin1
characters it fails with
```
qt.qpa.plugin: Could not find the Qt platform plugin "cocoa" in ""
This application failed to start because no Qt platform plugin could
be initialized. Reinstalling the application may fix this problem.

Abort trap: 6
```

To reproduce this, run (in bash):
```
python -m venv --clear ~/virt/šš
~/virt/šš/bin/python -m pip install PyQt5
~/virt/šš/bin/python -c "import PyQt5.QtWidgets as w; w.QApplication([])"
```

Explicitly setting the QT_PLUGIN_PATH to the correct location does work however
```
export QT_PLUGIN_PATH=$HOME/virt/šš/lib/python3.6/site-packages/PyQt5/Qt/plugins/
~/virt/šš/bin/python -c"import PyQt5.QtWidgets as w; w.QApplication([])"
```

The problem is that reading the "Paths/Prefix" in qt.conf with
QSettings (the way Qt does), produces a garbled path:
```
from PyQt5.QtCore import QSettings
s = QSettings(":qt/etc/qt.conf", QSettings.IniFormat)
print(s.value("Paths/Prefix"))
# -> .../virt/sÌ\x8csÌ\x8c/lib/python3.6/site-packages/PyQt5/Qt'
```

Reading the file with QFile
```
f = QFile(":/qt/etc/qt.conf")
f.open(QFile.ReadOnly)
print(bytes(f.readAll()).decode("utf-8"))
```
reveals that the contents are utf-8 encoded, however from
http://doc.qt.io/qt-5/qsettings.html#Format-enum

> QSettings will accept Latin-1 encoded INI files, but generate pure ASCII files, where non-ASCII values are encoded using standard INI escape sequences.

The `QSettings` does have a `setIniCodec` method, but it is not used
by Qt for parsing the qt.conf file.

Writing the path using QSettings
```
s = QSettings("a.conf", QSettings.IniFormat)
s.setValue("Paths/Prefix", ".../virt/šš/...")
```
produces:
```
[Paths]
Prefix=".../virt/\x161\x161/..."
```
which can then be correctly read back.

Unfortunately Qt does not provide a public interface for INI escape
sequence encoding. It is implemented in
http://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/io/qsettings.cpp?h=v5.11.2#n618

Tested on:
* macOS 10.11.6, Python 3.6, PyQt5 5.11.3
* KDE Neon 16.04 Python 3.5, PyQt5 5.11.3

Best wishes
   Aleš Erjavec