v3 is here!

Some of the major changes:

• Python 3 only
• Strictness by default: no more strict mode; `load` and `dump` return data
• Unknown field validation
• More intuitive datetime handling
• Improved Dict field with support for semi-structured data
• Generate schemas from dictionaries (see https://stevenloria.com/dynamic-schemas-in-marshmallow/ for use cases)
• Significantly improved performance

And lots more .

We've written a guide to upgrading your code: https://marshmallow.readthedocs.io/en/latest/upgrading.html

Thanks to all who contributed code, brainpower, and donations to make this release possible.