[BUG][python-nextgen] validation of list of enums doesn't work

Created by: t0mk

The template in https://github.com/OpenAPITools/openapi-generator/blob/18ef6ff96a00c5c3895d428679352b3af759fbdd/modules/openapi-generator/src/main/resources/python-nextgen/model_generic.mustache#L44

    def {{{name}}}_validate_enum(cls, v):
        {{^required}}
        if v is None:
            return v

        {{/required}}
        if v not in ({{#allowableValues}}{{#enumVars}}{{{value}}}{{^-last}}, {{/-last}}{{/enumVars}}{{/allowableValues}}):
            raise ValueError("must validate the enum values ({{#allowableValues}}{{#enumVars}}{{{value}}}{{^-last}}, {{/-last}}{{/enumVars}}{{/allowableValues}})")
        return v

.. is called also when "v" is a list, and it checks ["v1", "v2"] not in ["v1","v2"], and fails for a legit use case.

It should be sth like

    def {{{name}}}_validate_enum(cls, v):
        {{^required}}
        if v is None:
            return v

        {{/required}}
        if type(v) is list:
            for i in v:
                if i not in ({{#allowableValues}}{{#enumVars}}{{{value}}}{{^-last}}, {{/-last}}{{/enumVars}}{{/allowableValues}}):
                    raise ValueError("each list item must be one of ({{#allowableValues}}{{#enumVars}}{{{value}}}{{^-last}}, {{/-last}}{{/enumVars}}{{/allowableValues}})")
        else:
            if v not in ({{#allowableValues}}{{#enumVars}}{{{value}}}{{^-last}}, {{/-last}}{{/enumVars}}{{/allowableValues}}):
                raise ValueError("must be on of enum values ({{#allowableValues}}{{#enumVars}}{{{value}}}{{^-last}}, {{/-last}}{{/enumVars}}{{/allowableValues}})")

        return v

Example of enum list can be found in spec at: https://github.com/t0mk/metal-python/blob/05f4599ebe3ee42727bf5117403571a739fa52ba/metal_openapi.fixed.yaml#L2662

I'm a bit in a hurry but I hope I wrote it clear.