Use python slim images to run tests

[mayeut]

This should speed-up the tests a bit.

[codecov]

Codecov Report

Merging #308 (7939153) into master (0c59594) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #308   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
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  Files          20       20           
  Lines        1102     1102           
  Branches      237      237           
=======================================
  Hits         1006     1006           
  Misses         54       54           
  Partials       42       42           

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[mayeut]

@lkollar,

This was mostly to check that TravisCI builds were still triggered following an issue on the Travis CI pypa account.
Nevertheless, I still think it's a good thing to use the slim image.

[auvipy]

slim is better