Add support for compile-time configuration of default TZPATH

[pganssle]

This is a bit harder to backport, because in CPython we are able to store this information in sysconfig. I think we can generate a file at build time that we use preferentially over the current default values should be enough; it can be populated by an environment variable during the build.

Testing it may be mildly annoying.

CC: @mariocj89

[mariocj89]

Do you mean a file within the package which contains the paths to lookup? That sounds good, as it will be easy to patch.

Ideally, I'd expect distributions to just consume the sdist and set the environment variable when repackaging or just use the runtime env var if they need some environment-specific setup.

Sounds great! I'm happy to push a PR for this if you want.

[pganssle]

Do you mean a file within the package which contains the paths to lookup? That sounds good, as it will be easy to patch.

There would be no need to patch anything like this, it would be configurable at build time.

Ideally, I'd expect distributions to just consume the sdist and set the environment variable when repackaging or just use the runtime env var if they need some environment-specific setup

Currently we don't have any environment variable to set during the build, that's what this issue is about. In CPython, you can do ./config --with-tzpath=/usr/share/zoneinfo, but in this case if you don't have PYTHONTZPATH set, it always searches a pre-defined set of paths.