Fix union type hint

Author: ottmar-zittlau

modules/openapi-generator/src/main/resources/python-pydantic-v1/model_generic.mustache

@@ -126,7 +126,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
         return json.dumps(self.to_dict())
 
     @classmethod
-    def from_json(cls, json_str: str) -> {{^hasChildren}}{{{classname}}}{{/hasChildren}}{{#hasChildren}}{{#discriminator}}Union({{#children}}{{{classname}}}{{^-last}}, {{/-last}}{{/children}}){{/discriminator}}{{^discriminator}}{{{classname}}}{{/discriminator}}{{/hasChildren}}:
+    def from_json(cls, json_str: str) -> {{^hasChildren}}{{{classname}}}{{/hasChildren}}{{#hasChildren}}{{#discriminator}}Union[{{#children}}{{{classname}}}{{^-last}}, {{/-last}}{{/children}}]{{/discriminator}}{{^discriminator}}{{{classname}}}{{/discriminator}}{{/hasChildren}}:
         """Create an instance of {{{classname}}} from a JSON string"""
         return cls.from_dict(json.loads(json_str))
 
@@ -256,7 +256,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
         return _dict
 
     @classmethod
-    def from_dict(cls, obj: dict) -> {{^hasChildren}}{{{classname}}}{{/hasChildren}}{{#hasChildren}}{{#discriminator}}Union({{#children}}{{{classname}}}{{^-last}}, {{/-last}}{{/children}}){{/discriminator}}{{^discriminator}}{{{classname}}}{{/discriminator}}{{/hasChildren}}:
+    def from_dict(cls, obj: dict) -> {{^hasChildren}}{{{classname}}}{{/hasChildren}}{{#hasChildren}}{{#discriminator}}Union[{{#children}}{{{classname}}}{{^-last}}, {{/-last}}{{/children}}]{{/discriminator}}{{^discriminator}}{{{classname}}}{{/discriminator}}{{/hasChildren}}:
         """Create an instance of {{{classname}}} from a dict"""
         {{#hasChildren}}
         {{#discriminator}}