Add testing and document support for Python 3.7 & pypy3

[jdufresne]

No description provided.

[davidism]

What's the difference between "pypy3.5-6.0" and "pypy3" on Travis? I've just been using "pypy3".

[davidism]

I tried to do dist: xenial globally for Werkzeug a month ago and it didn't work. Guess something changed.

[jdufresne]

To get Python 3.7, I switched the dist to xenial. Details on the release here: https://blog.travis-ci.com/2018-11-08-xenial-release

Unlike trusty, xenial requires an explicit version for pypy. See the issue report here: https://travis-ci.community/t/pypy-2-7-on-xenial/889

On trusty, pypy3 is just an alias.

[jdufresne]

I tried to do dist: xenial globally for Werkzeug a month ago and it didn't work. Guess something changed.

Prior to the release, projects were required to add sudo: required to get xenial to work. But this has been resolved. Details here: travis-ci/travis-ci#9815

[davidism]

Cool! I need to follow Travis announcements more closely. All the other Pallets projects need updating if you feel like making more PRs.