[ci] separate linting task (fixes #37)

[jameslamb]

this PR tries to speed up continuous integration with the following changes:

• don't test on Python 3.5
• run all linting tasks (and install linting-specific libraries) in a single build

[codecov-commenter]

Codecov Report

Merging #178 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #178   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          320       320           
=========================================
  Hits           320       320           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update eecee4f...19c0c23. Read the comment docs.

[jameslamb]

It seems like it cut the time at least a little! Nothing huge, but big enough to be worth it, I think.

previous build

this PR