Allow the brand new 4.x release of pyyaml

[asottile]

the most important "breaking" change of pyyaml 4.x is the move to "safe by default" -- I browsed awscli's usage of pyyaml and it looks like the safe loader is already being used

[asottile]

(I initially ran into this from the following warning from pip):

awscli 1.15.46 has requirement PyYAML<=3.12,>=3.10, but you'll have pyyaml 4.1 which is incompatible.

[codecov-io]

Codecov Report

Merging #3414 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #3414   +/-   ##
=======================================
  Coverage     94.4%   94.4%           
=======================================
  Files          169     169           
  Lines        13223   13223           
=======================================
  Hits         12483   12483           
  Misses         740     740

---

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 ce93d2a...f5611b0. Read the comment docs.

[hugovk]

👍

This should also fix problems installing awscli on Python 3.7: yaml/pyyaml#126.

(Python 3.7 is due for release today, 2018-06-27: https://www.python.org/dev/peps/pep-0537/)

How about adding 3.7-dev to .travis.yml and py37 to tox.ini in this PR?

[asottile]

the problem with travis-ci's 3.7-dev is it's 3.7.0a4 which is quite different from the 3.7.0 final that's going to be released this week. most of this is due to travis being stuck on trusty (3.7.0 will require libssl 1.0.2, whereas trusty only has 1.0.1)

[asottile]

well it got pulled, wonderful!

[asottile]

I've opened #3430 for the just-released 3.13