Interpret naive datetimes as UTC

[gadomski]

Related Issue(s):

• Closes #644

PR Checklist:

• [x] Code is formatted
• [x] Tests pass
• [x] Changes are added to the CHANGELOG

[gadomski]

Dunno when we do a changelog, but it it's at release time we'll want to call this out prominently.

Good point — I hadn't thought about that. Do you think it's enough of a change to warrant a "MAJOR" release (i.e. v0.8.0) to get people's attention?

[TomAugspurger]

Maybe. I'm a fan of @jacobtomlinson's effver. Getting users to pause and think about whether they're using naive datetime objects seems worthwhile.