TST: Add python 3.7 testing

[thequackdaddy]

I think this will get python 3.7 testing on patsy

[codecov-io]

Codecov Report

Merging #140 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   98.96%   98.96%   +<.01%     
==========================================
  Files          30       30              
  Lines        5582     5585       +3     
  Branches      775      775              
==========================================
+ Hits         5524     5527       +3     
  Misses         35       35              
  Partials       23       23

Impacted Files	Coverage Δ
patsy/constraint.py	98.01% <100%> (+0.01%)	⬆️

---

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 e141c8a...1869503. Read the comment docs.

[thequackdaddy]

I also added a try ... except around the import collections.abc from Mapping as that got moved.

[njsmith]

Apparently I misremembered the syntax, and language: none actually means "ruby". The actual no-language images are minimal and generic: https://docs.travis-ci.com/user/languages/minimal-and-generic/

I just tried switching it to minimal, let's see what Travis does...

[astrojuanlu]

Any chance to release these changes?