Sanity check minimum required version of PyQt5

pyblish-qml requires at least 5.7.1, but doesn't do a good job in letting the user know he's got an older version, e.g. 5.3.

Instead, it says:

```bash
file:///C:/pyblish-qml/pyblish_qml/qml/app.qml:9:1: module "QtQuick.Controls" version 1.3 is not installed
```

And what version of QML is related to which version of Qt??