Inlcude tests in PyPI tarball

[dotlambda]

The tests are useful for downstream packagers and do not hurt normal users (because they are usually not run).

[codecov]

Codecov Report

Merging #323 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
- Coverage   87.81%   87.74%   -0.08%     
==========================================
  Files          19       19              
  Lines        1412     1412              
==========================================
- Hits         1240     1239       -1     
- Misses        172      173       +1

Impacted Files	Coverage Δ
sigal/plugins/watermark.py	90.69% <0%> (-2.33%)	⬇️

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[saimn]

Sorry for the delay!
Right now I prefer not to accept this because I'm not completely sure of the license of all the images present there. It has been on my todo list to check this since a long time but ...
Also I'm not convinced that it is useful to add ~4Mb to the package, as the majority of the users will not run the tests this way. It's not huge but if every package does the same the site-packages is quickly growing in size for no real gain.

[saimn]

Ok, now that I checked the included images and video (#340), let's move forward and merge this. Thanks @dotlambda !