It is sufficiently rare that we have to touch this stuff that I always have to re-discover where/how it is done. And it is impractical to grep for "bitcoin:" or "lightning:".
Putting this "master list" comment very close to the BITCOIN_BIP21_URI_SCHEME variable seems like a good spot - at least this is where I would look for it first.
urllib.parse needs to be explicitly imported for e.g. urllib.parse.urlparse to work.
I think this was not erroring only because urllib.parse is imported in other modules.