Install tzdata in the test environment

`ubuntu-latest` now resolves to Ubuntu 26.04, whose system zone files
leave out the legacy `US/*` names that the timezone list relies on, so
the suite fails at collection with "Invalid timezone US/Hawaii".
`zoneinfo` falls back to the `tzdata` package for names the system
does not have, which keeps the tests independent of the runner image.

Author: gil-doesit

tox.ini

@@ -11,6 +11,7 @@ python =
 [testenv]
 deps =
     pytest
+    tzdata
 commands =
     pytest {posargs}