dedupe union typestrings

[straz]

The codegen algorithm has a bug when it descends into UnionProperties with an anonymous object as an inner type. Example:

    FieldValueWithResolution:
      oneOf:
        - type: string
        - type: boolean
        - type: number
        - type: array
        - type: object

The codegen used to be forgiving because it would always return an instance of FieldValueWithResolution(additional_properties=...) without any attempt to get the nested structure correct.
Now that we are generating typevars, the broken recursion is generating children with the same name
as the parent, which results in this output:

FieldValueWithResolution = Union[str, bool, float, List[str], FieldValueWithResolution, UnknownType]

A correct fix would overhaul the recursive parser, much more than we can afford to do for now.
A reasonable quick fix is to add a filter on the get_type_strings() method to filter out children with the same name as the parent:

FieldValueWithResolution = Union[str, bool, float, List[str], UnknownType]

[alexrejto]

🚀