Address changes to Schema().load and Schema().dump in marshmallow v3

[prkumar]

After marshmallow 3.0, Schema().load and Schema().dump don't return a (data, errors) tuple any more. Only data is returned.

See: https://marshmallow.readthedocs.io/en/3.0/upgrading.html

This tiny PR accommodates these changes for our examples and integrations with marshmallow.

[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #109   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          25     25           
  Lines        1392   1395    +3     
  Branches      131    131           
=====================================
+ Hits         1392   1395    +3

Impacted Files	Coverage Δ
uplink/converters/marshmallow_.py	100% <100%> (ø)	⬆️

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