[commands] Fix typing.Union converters for 3.7

Guido please don't break this

discord/ext/commands/core.py

@@ -257,7 +257,12 @@ class Command:
         if converter is bool:
             return _convert_to_bool(argument)
 
-        if type(converter) is typing._Union:
+        try:
+            origin = converter.__origin__
+        except AttributeError:
+            pass
+        else:
+            if origin is typing.Union:
                 errors = []
                 for conv in converter.__args__:
                     try: