v3 adds generic type info to schema classes/properties/items

[spacether]

• removes double underscore private prefix from apiresponse/mediatype/content classes
  • this was done to fix type hint access in vscode
• uses generics in schemas so IDEs can see instance and property base types
  So now when models are instantiated, they will show what class they are based on
  Like so:

my_model = MyModel(some_str='a')
# my_model is type MyModel[frozendict.frozendict] in the vscode IDE
# my_model.some_str is type SomeStr[str] in the vscode IDE

• keep the mixin classes for ide methods (needed to show possible properties)
• make Schema generic
• when making classes always define new so the generic can be seen
• update properties to add the generic type
• update ref props to add generic
• fix UnsetAnyTypeSchema not being used in AbstractStepMessage
• update get_item property with type
• update get_item array with type
• fix __new__ type checking warning for dictshemas
• update UnsetAnyType type hints to be AnyType because it will always be true and UnsetAnyType will not
• add __init__ method to AnyTypeSchema
• make sure that the new method is always written for custom classes
• update response body to be typed
• update response content mediatype to be typed
• update header schema to be typed
• update header content mediatype to be typed
• update parameter schema to be typed
• update parameter content mediatype to be typed

Out of scope future improvement.

• Use typing.Never in NotAnyType
• Changing ref schemas to type aliases. This is not work because assignment needs the prop to be there
  • perhaps it would work if it was in annotations only

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