Maintenance

[dopplershift]

There's a ton of things we need to clean up on Siphon:

• [x] Retire Python 3.5 build and add 3.7
• [x] Update Travis to use python 3.7-dev for "bleeding edge" builds
• [x] Update AUTHORS.txt for any new code contributors
• [x] Update year in LICENSE
• [x] Update .mailmap as needed to keep contributors unique
• [ ] Remove any spurious warnings from tests/doc/example builds. I thought when building docs locally I was seeing our own deprecation warnings, which is silly.
• [x] Update CODEOWNERS to replace jrleeman with zbruick

Also fair game to bring in anything else useful from Metpy.

This is a great way to find authors: git log --format='%aN %aE' | sort -u

I'm putting this here so I don't forget, but it's fair game if someone is exceedingly motivated. Just make sure you drop a note below if you happen to pick it up so we don't collide.

[zbruick]

I'll work on this - I think all of them should be straight forward for me, except removal of spurious warnings possibly.

[zbruick]

#271 addresses 7/8 of the issues above. Only thing that remains is the removal of spurious warnings

[zbruick]

Note that the code of conduct will be covered by a default Unidata CoC soon.