Add track-changes-reason audit attribute

[seadowg]

This adds the track-changes-reasons as defined in this spec.

[seadowg]

I don't understand the failures at all

[codecov-io]

Codecov Report

Merging #400 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
+ Coverage   82.45%   82.48%   +0.03%     
==========================================
  Files          23       23              
  Lines        3265     3271       +6     
  Branches      764      766       +2     
==========================================
+ Hits         2692     2698       +6     
  Misses        432      432              
  Partials      141      141

Impacted Files	Coverage Δ
pyxform/constants.py	100% <100%> (ø)	⬆️
pyxform/xls2json_backends.py	82.45% <100%> (ø)	⬆️
pyxform/xls2json.py	78.41% <100%> (+0.16%)	⬆️

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[lognaturel]

Looks good to me.

@ukanga, can you please help with the black failure? I can't tell whether black is somehow unsupported for those python versions all of a sudden or whether there are real problems.

[seadowg]

For reference I did (from outside the repo dir):

pip install black
black --target-version=py27 pyxform

I'm using Python 3.7.4 and black 19.10b0.

[seadowg]

@ukanga @lognaturel fixed this by using the same version of black that's in requirements.pip. I'll add a couple fo commits to the docs around this.

[lognaturel]

I extra approve these. No comments and there's nothing controversial in here so let's merge so we can focus on the trickier open PRs.