tzdata removed all US/* aliases which broke our tests big time. At
first I thought this was a bug in pytz, but nope, it's just doing what
it's doing and failing properly.

So instead of fighting changes, just yell loudly at our users that
this is happening (something which tzdata seemed it didn't need to do)
and fix our default configuration and tests.

We also Suggests tzdata-legacy in a hint at how broken things might be
without. But we don't go as far as making that Recommends to go with
the flow, even though we think it's kind of bonkers to break such a
commonly used alias.