D992 introduced content negotiation for flask-based APIs using the negotiate python package.
https://github.com/nickstenning/negotiate
This later is very small with no activity for years, so we should just integrate the part of the code we use un swh.core instead of adding yet another (with no debian package) dependency... It looks to be under the MIT license, so it should be okay.
Note that I've modified the original file, so I'm not sure whether we should keep it (swh/core/api/negotiate.py) under its original MIT license (in which case the debian package's copyright file should be modified) or if we put it under the swh.core's LGPL one.
It's fine either way, as MIT is upward compatible with GPL (not LGPL, AFAIK) of swh.core.
I'd say to just leave it as MIT in the file itself, as it's good practice to contribute under the same license of the target project.