Adding new tags crashes Picard with Qt < 5.10

Adding a new tag when using Qt < 5.10 crashes Picard with:

Traceback (most recent call last):
File "/snap/picard/x5/lib/python3.6/site-packages/picard/ui/edittagdialog.py", line 141, in edit_value
if item and not self.value_list.isPersistentEditorOpen(item):
AttributeError: 'QListWidget' object has no attribute 'isPersistentEditorOpen'
Abgebrochen (Speicherabzug geschrieben)

The issue was introduced in a6b35dd with the fix for PICARD-1860.

This is relevant as the snap version currently uses Qt 5.9 (due to the kde-neon which is the basis for most Qt apps and dependency issues when bundling a newer version). Also we claim compatibility with Qt >= 5.7.1.