Add _repr_html_ to tables

[benjeffery]

Closes #512

This method is called by jupyter notebooks to display nice-looking tables. Here's a screenshot:

[benjeffery]

No idea why the docs are broken, they build locally. Will fix the CI tomorrow, but def not the fault of these changes.

[jeromekelleher]

Very tidy, I like it!

[jeromekelleher]

One to add to the CHANGELOG too.

[benjeffery]

@jeromekelleher changelog added (fixed up a few other entries too)

[jeromekelleher]

Looks like the docs build is failing because of a new sphinx version. Sigh.

[benjeffery]

Things like this are why deps should be version-pinned, at least for the CI. At the moment the CI isn't deterministic. We can use https://dependabot.com/ to keep the pinned versions up-to-date. It submits PRs automatically and we would see the CI break there instead of here.

[benjeffery]

@jeromekelleher I'm fixing the docs - will be in a diff PR.

[codecov]

Codecov Report

Merging #514 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   86.82%   86.84%   +0.01%     
==========================================
  Files          21       21              
  Lines       15979    15996      +17     
  Branches     3105     3109       +4     
==========================================
+ Hits        13874    13891      +17     
  Misses       1055     1055              
  Partials     1050     1050              

Flag	Coverage Δ
#c_tests	87.75% <100.00%> (+0.01%)	⬆️
#python_c_tests	90.55% <100.00%> (+0.02%)	⬆️
#python_tests	99.18% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
python/tskit/tables.py	99.80% <100.00%> (+<0.01%)	⬆️

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