fix PySide2 dependency

[AydinChavez]

This fixes following error during pip install:

ERROR: Could not find a version that satisfies the requirement PySide2==5.15.2 (from webui) (from versions: 0.0.0a1, 5.11.0, 5.11.1, 5.11.2, 5.12.0, 5.12.1, 5.12.2, 5.12.3, 5.12.4, 5.12.5, 5.12.6, 5.13.0, 5.13.1, 5.13.2, 5.15.2.1)
ERROR: No matching distribution found for PySide2==5.15.2

[eissar]

This fixed the issue for me. However I had to change a line in
flask.sessions line 14 from collections import MutableMapping
to:
if sys.version_info[:2] >= (3, 8): from collections.abc import MutableMapping else: from collections import MutableMapping
so that it would build

the newest flask module seems to be updated with that change on the github page