switch from PyQt6 to PySide6

PySide6 uses LGPL instead of GPL, which is much nicer to work with.

raven/pluginregistry.py

@@ -1,9 +1,6 @@
 from types import ModuleType
 from typing import Dict, Optional
 from inspect import isclass
-from pkgutil import iter_modules
-from pathlib import Path
-from os.path import dirname
 from importlib import import_module
 from inspect import signature
 from raven.pluginbase import PluginBase