Setup CI coverage for tzif library and tzdata package

[allenporter]

Is your feature request related to a problem? Please describe.

The current CI uses whatever environment python has setup, and instead we want an environment that simulates no local zoneinfo using tzdata package.

Describe the solution you'd like

We may want to run in a mode that acts like zoneinfo is not available, and ensures that tzdata package is installed. I think we first check zoneinfo files before checking tzdata, so this may be able to be done with a fixture with different patches for each test mode.

Describe alternatives you've considered

Alternatives could include running separate images for testing at a lower level (e.g. the os image handles this). Windows may be the other interesting test case to consider as the target of tzdata, which probably wouldn't run in CI.

[allenporter]

This package now always installs tzdata and prefers that first, before looking at system timezone information rather than assuming anything based on the install state of the system.