Add support for PEP 563 (#356)

[joernheissler]

If a module imports annotations from __future__, postpone the evaluation
of annotations like it is done for annotation strings.

Closes #356

[joernheissler]

Why does a "nightly" use cpython version "3.7.0a4+"? :-(
Should 3.7 alphas and betas be supported? Or can the "nightly" be upgraded to a more recent python version?

[sigmavirus24]

@joernheissler we don't control what Travis CI packages as "nightly". I suspect to use a more recent nightly we'd need to discern how to get Travis to give that to us.

[asottile]

@joernheissler "nightly" for trusty is the last version that supports libssl1.0.0 -- if you want actual nightly you'll need to use dist: xenial. Here's some more information: https://github.com/deadsnakes/travis-ci-python3.7-example#travis-ci-python37-example

[joernheissler]

"update 2018-10-15: sudo: required is no longer necessary" is the good news. Let me try that.

[asottile]

you'll have to remove sudo: false (it's the default afaik) for some reason. travis is weird 🤷‍♂️

[joernheissler]

Thanks @asottile, but more trouble as you can see on #379 :-(

[asottile]

If you switch to matrix you can just turn on xenial for 3.7 / nightly: example

the python3.6 env there is using the default (trusty) dist, where as the 3.7 job is using xenial

[joernheissler]

This PR now includes #369 and #379 to make Travis happy. I hope it's fine to do it this way.

[sprt]

Hey, could this be merged?

[karolyi]

voting + 1 for merging this, as, python3.7 spreads more and more over time.

[sigmavirus24]

Hey, could this be merged?

There's a merge conflict which should probably be resolved before we merge it or otherwise

[joernheissler]

@sigmavirus24, I'm happy to fix any merge conflicts, but I guess #369 needs to be merged first.

[senciucserban]

Any news on this?

[nmatthews-asapp]

Any updates?

[joernheissler]

I rebased this PR and #379.

[asottile]

lg2m