PY-18739 Union types are generated in PEP 484 compatible format

python/pydevSrc/com/jetbrains/python/debugger/PySignature.java

@@ -120,7 +120,7 @@ public class PySignature {
         return myTypes.get(0);
       }
       else {
-        return StringUtil.join(myTypes, " or ");
+        return "Union[" + StringUtil.join(myTypes, ", ") + "]";
       }
     }