Chris, I see no evidence in the *incomplete* traceback not posted here that there is a bug in mock.  Mock calls "hasattr(obj, '__func__') and apparently flask, called via werkzeug, answers 'RuntimeError'.  The overt bug is that calling werkzeug.local.(type(obj)?).__getattr__ fails by doing something other than "return the (computed) attribute value or raise an AttributeError exception".
https://docs.python.org/3/reference/datamodel.html#object.__getattr__