Update marshmallow to 2.18.1

[ivan-artezio]

Marshmallow 2.18.0 raises ChangedInMarshmallow3Warning for nested schemas which is annoying when running tests and clogs the logs. Marshmallow should be updated to 2.18.1 where this issue was fixed.
https://marshmallow.readthedocs.io/en/3.0/changelog.html
marshmallow-code/marshmallow#1136

[multimeric]

I would say Marshmallow needs to be updated to 3.X

[PATolk]

I needed to upgrade Marshmallow to 3.2.1
and now I get an error
File "/lib/python3.6/site-packages/flask_rest_jsonapi/resource.py", line 131, in get
result = schema.dump(objects).data
AttributeError: 'dict' object has no attribute 'data'

[multimeric]

Yes, the library has to be changed to make it Marshmallow 3 compatible. I've fixed this in my PR: #172. However it hasn't yet been merged.

[auvipy]

this can be closed now