add support for working with pytz object field

[MilanPecov]

When a pytz object is serialized it fails with this error

TypeError: Object of type Africa/Abidjan is not JSON serializable and this issue has already been reported (for example #248)

The current workaround is to modify the methods in the pytz serializer field, but it becomes more complicated when other third party libraries in the project are also broken because of this.

So I think it is reasonable to assume that because any bigger real project will require working with a timezone, it's better to fix the problem here instead of making modifications on various fields and libraries. And it's just a few lines fix.

Also I'm not sure if this change requires tests and if they are needed where should I add them?

Thanks

[MilanPecov]

I would also label this a bug because when a pytz object is serialized the docs are not working at all

[JoelLefkowitz]

@MilanPecov I agree this is good idea. If the user doesn't want the pytz objects to be parsed to strings they still have the option to modify the serializer. If there are other common field types that are limiting json encoding then we may need a separate method to handle them.