Require python 3.6

[benjeffery]

Fixes #103
I've added 3.7 to travis too, as although it's in appveyor the travis builds happen in parallel so there's no harm.

[codecov]

Codecov Report

Merging #114 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #114   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files           7        7           
  Lines         996      996           
  Branches       64       64           
=======================================
  Hits          993      993           
  Misses          1        1           
  Partials        2        2           

Flag	Coverage Δ
#C	99.69% <ø> (ø)
#python	99.31% <ø> (ø)

Impacted Files	Coverage Δ
c/kastore.c	100.00% <0.00%> (ø)

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[jeromekelleher]

Lgtm, thanks.