How to install an spkg (the one for GRDB Fano polytopes)?

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This is an "old-style" Sage SPKG, and those are no longer supported.

The way to try to install, which would be unsuccessful, is to download the file from http://coates.ma.ic.ac.uk/grdb_polyto..., then run sage -i /path/to/grdb_polytopes-0.1.spkg. This will fail with the message

Error: package '/Users/palmieri/Downloads/grdb_polytopes-0.1.spkg' not found
Note: if it is an old-style package, installing these is no longer supported

I don't see any easy way to work around this. You can still look at the data: this spkg file is just a compressed tarball, so you can download it and then run tar xf grdb_polytopes-0.1.spkg and then look at the data files in grdb_polytopes-0.1.spkg/src.