Remove py27 CI jobs
The start of a looooooooong process to get rid of the py2 compat code.

I kept most of the CI jobs that used to be 2.7 at the lower end of the various supported versions, with 2x 2.7->3.5, 1x 2.7->3.6 and only one real upgrade from 2.7->3.7 (first CI job for 3.7 & windows).

@jreback @TomAugspurger 
