Theming ControlPanel broken: hostnameBlacklist WrongType in Py3

[MrTango]

2020-04-23 11:06:56,362 ERROR   [Zope.SiteErrorLog:252][waitress] 1587629216.36216240.9807038548787032 http://localhost:8080/Plone6mig/@@theming-controlpanel
Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module plone.app.theming.browser.controlpanel, line 56, in __call__
  Module plone.app.theming.browser.controlpanel, line 200, in update
  Module plone.registry.recordsproxy, line 60, in __setattr__
  Module plone.registry.registry, line 51, in __setitem__
  Module plone.registry.record, line 82, in _set_value
  Module zope.schema._bootstrapfields, line 291, in validate
  Module zope.schema._field, line 814, in _validate
zope.schema._bootstrapinterfaces.WrongContainedType: ([WrongType(b'127.0.0.1', <class 'str'>, '')], 'value')

[MrTango]

@mauritsvanrees
fixed in d6db78d
changelog entry added here: ac28556

in case we will release this changes only for 6.0, we need a backport for Plone 5.2 with the fix.