Converters cannot convert datetime.timedelta to float.

hello devs.
During a conversation about an issue of graphql-core @jkimbo found a possible error

https://github.com/graphql-python/graphene-django/blob/5051d3bb617ce6977eebc023c958bd10de91fe91/graphene_django/converter.py#L111-L113

raising something like ```{
"message": "float() argument must be a string or a number, not 'datetime.timedelta'"
}```

you can check https://github.com/graphql-python/graphql-core/issues/150 for more info.