Adopt Python 3.7

[gaborbernat]

• add Python 3.8 dev tests as failure allowed
• Python 3.7 tests are no longer allowed to fail

[codecov]

Codecov Report

Merging #866 into master will increase coverage by <1%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #866    +/-   ##
======================================
+ Coverage      92%    92%   +<1%     
======================================
  Files          12     12            
  Lines        2331   2331            
  Branches      408    408            
======================================
+ Hits         2155   2156     +1     
  Misses        110    110            
+ Partials       66     65     -1

[asottile]

this is likely blocked on

• pyyaml making a release
• appveyor having python3.7 installed

[gaborbernat]

I'll move fix-lint to Python 3.6 (we'll update it to 3.7 after yaml/pyyaml#193). And for now, not run Windows test for Python 3.7 (we'll add it in a new PR once appveyor/ci#2475 is released).

With these three we can go ahead. Also defined stages to not run tests if the linting fails.