Marshmallow-mongoengine

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- Remove six usage to avoid `undeclared dependency <https://github.com/MongoEngine/marshmallow-mongoengine/issues/22>`_

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- Use metadata argument to avoid `marshmallow deprecation warning <https://marshmallow.readthedocs.io/en/stable/changelog.html#id15>`_

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- Add support for Date field
- Fix LineStringField support

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- Allow loading embedded reference

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- Misc bugfixes

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- Add support for marshmallow v3
- Remove support for python2 since mm-v3 needs >= p35.
- Removed support for toasted since doesn't support mm-v3