Warren_2020 downloads fail on Windows

[Jost Migenda]

A project student running snewpy on Windows discovered an error trying to download Warren_2020 files; where trying to run the code from the sample notebook resulted in:

HTTPError: 404 Client Error: Not Found for url: https://github.com/SNEWS2/snewpy-models-ccsn/raw/v0.2/models/Warren_2020/stir_a1.23%5Cstir_multimessenger_a1.23_m10.0.h5

Note the %5C in the URL. This is percent encoding for the \ character (instead of the / character that would make this a valid URL).
I think I’ve got a fix ready; but I’ll also see if we can add CI tests under Windows, to avoid things like this in the future.