Added the "YAML" extra to apispec

[rwb27]

We use the yaml functionality of apispec, which
means we should include it as a dependency.
I've also updated the lock file with the new dependencies.

Closes #228

[codecov]

Codecov Report

Merging #232 (345d544) into master (24d1f77) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #232   +/-   ##
=======================================
  Coverage   90.54%   90.54%           
=======================================
  Files          40       40           
  Lines        1745     1745           
  Branches      277      277           
=======================================
  Hits         1580     1580           
  Misses        110      110           
  Partials       55       55           

Flag	Coverage Δ
unittests	90.54% <ø> (ø)

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