Update to Marshamallow 3, also upgrade webargs flask_apispec

[adelavega]

We're using a fairly old version of Marshmallow, so I'm upgrading to v3, and upgrading webargs flask_apispec as well.

In the process, I also updated the Dockerfile to buster from stretch.

[adelavega]

@rwblair I think I was able to upgrade to marshmallow 3! It wasn't as bad as I thought fortunately, just a couple of tricky changes.

Do you mind testing this against the frontend locally on your dev machine? If it looks good I'll test on live server.

[codecov-commenter]

Codecov Report

Merging #816 into master will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
- Coverage   83.25%   83.16%   -0.10%     
==========================================
  Files          63       63              
  Lines        2902     2886      -16     
==========================================
- Hits         2416     2400      -16     
  Misses        486      486              

Impacted Files	Coverage Δ
neuroscout/tests/api/test_predictor.py	100.00% <ø> (ø)
neuroscout/api_spec.py	100.00% <100.00%> (ø)
neuroscout/models/dataset.py	92.10% <100.00%> (ø)
neuroscout/resources/analysis/endpoints.py	95.17% <100.00%> (ø)
neuroscout/resources/analysis/reports.py	46.66% <100.00%> (+0.44%)	⬆️
neuroscout/resources/dataset.py	100.00% <100.00%> (ø)
neuroscout/resources/predictor.py	89.15% <100.00%> (ø)
neuroscout/resources/run.py	100.00% <100.00%> (ø)
neuroscout/resources/task.py	100.00% <100.00%> (ø)
neuroscout/resources/user.py	74.02% <100.00%> (ø)
... and 8 more

---

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[adelavega]

Oh, right. The main thing that might still not work is the multi-part file upload. Need to test that out still...

[adelavega]

Fixed the file upload by doing a double Marshmallow Schema that ignores extra arguments. This accept all arguments, but still appropriately documents them.