Some IANA Timezones are not accepted

[aladakepri]

https://en.wikipedia.org/wiki/List_of_tz_database_time_zones

• The "Europe/*" IANA codes do not appear to work when passed to APISession.create_site(site)
• The "US/*" IANA codes do not appear to work.
• The "Etc/*" appear to work
• The "America/*" codes appear to work

The trace, when timezone='Europe/Tirane', for example:

The "US/" codes are just links to "America/" codes, so this isn't a problem, but the "Europe/" codes are canonical, and offer no working replacement that accounts for daylight savings, etc.

Appears to be the source of the problem: https://github.com/SolarArbiter/solarforecastarbiter-api/blob/280800c73eb7cfd49029462b352887e78f1ff91b/sfa_api/utils/validators.py#L47