tests: update travis config

[sebix]

fix build: pytest needs to be updated
remove 3.3, it is EOL
add 3.7

[coveralls]

Coverage decreased (-0.1%) to 99.88% when pulling bb7b71b on sebix:fix-travis into 146f82d on textile:master.

[codecov-io]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #68   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        1213   1213           
  Branches      226    226           
=====================================
  Hits         1213   1213

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[ikirudennis]

Ever feel like you just don't know how github expects you to handle pull requests? I tried checking out a branch that I thought would track to this pull request, but no. So I'll abandon that one and bring the tiny change over here by editing in the browser. As soon as this passes, it should be good to go. We'll see if I can figure out the right way to do this for the next pull request.