Don't define properties and proprety setters for fields in Thrift Unions.

Summary:
Thrift Unions do not provide field-level attributes, and rather require using the explicit getter and setter methods, unlike structures.

This can generate confusion in the users as perfectly valid types lead to runtime failures.

Reviewed By: joshkehn

Differential Revision: D52565429

fbshipit-source-id: 7a33047527ec36f33ca24a879e22902acdd038c5

Author: Flameeyes

thrift/compiler/generate/templates/pyi/ttypes.pyi.mustache

@@ -60,6 +60,7 @@ class {{struct:name}}{{#struct:exception?}}(Thrift.TException){{/struct:exceptio
     ) -> None:
         ...
 
+    {{^struct:union?}}
     {{#struct:fields}}
     {{#field:type}}
     @__property__
@@ -69,6 +70,7 @@ class {{struct:name}}{{#struct:exception?}}(Thrift.TException){{/struct:exceptio
     def {{field:py_name}}(self, value: {{> common/OptionalFieldType}}) -> None: ...
     {{/field:type}}
     {{/struct:fields}}
+    {{/struct:union?}}
 
     {{#struct:union?}}
     def getType(self) -> int: ...