Figure out why CODEOWNERS is not working

[brettcannon]

E.g. #1918 should have been assigned to Mark. And every PR should be assigned to someone thanks to our wildcard.

Probably need to bisect the file by commenting out lines to see if there's some specific section that isn't working (e.g. a bad username is breaking the whole file).

[JelleZijlstra]

There might be a few issues:

• A stray line saying "Daniel Urban", fixed in Remove a bad line from CODEOWNERS #1922
• I ran https://github.com/mszostok/codeowners-validator and got some errors like:

    [err] line ...: User "@..." is not a member of the organization

Which is weird, since those people should definitely be in the Python org. Maybe it's just because I didn't give this tool my GitHub token and some of these people show up in https://github.com/orgs/python/people?query= as "private" for some reason.

• Another run found:

    [err] line 252: "pep0418/" does not match any files in repository

[JelleZijlstra]

I believe #1925 fixes it.

I'm worried about this breaking again silently in the future. If we ever remove a directory mentioned in CODEOWNERS, or (I believe) if anyone mentioned there leaves the python org, the file will silently stop working. It might be worth adding the GitHub action from the validator tool linked above to make sure the file remains valid. It would be good if GitHub provided some feedback directly if the file is invalid.