For the upcoming 2.7.0 release, I think it's important for at least tz.gettz to return the same zone for the same inputs unless otherwise specified.

This PR makes tzstr a singleton, adds caching to tz.gettz and adds the .instance alternate constructor for tzoffset and tzstr.

I did not bother with a .instance alternate constructor for tzutc, because the semantics of tzutc are not affected by inter- and intra-zone comparisons (technically true of tzoffset as well, too).