1.24 fails to build in 2021

While working on reproducible builds for openSUSE, I found that our python-skyfield-1.24 package fails tests in 2021 because calls to load.timescale without builtin=True are used a lot:

grep load.timescale skyfield/tests/*.py|grep -v builtin=True | wc -l
107

and these try to pull updated data from ftp://cddis.nasa.gov and that fails in our builds, because we disable the network to ensure reproducible results.

See also https://reproducible-builds.org/ for why this matters.

[bmwiedemann]

python-skyfield-1.26 builds fine in my tests even in 2035.

[brandon-rhodes]

Regarding the handling of expired files: I think it would still be warranted to print a warning or even throw an error (if so desired by configuration) for outdated local files.

That’s a good idea! Let’s leave this issue open, and tomorrow I’ll close it with a commit that adds documentation showing both approaches: code folks can paste into their app that prints a warning, and code that will actually throw an error, if a file is outdated. That way folks can have give their apps either grade of protection without needing to go look up the Python maneuvers required.