Adding a non-metadata installer-only `dev-dependencies` table to pyproject.toml

[ofek (Ofek Lev)]

@brettcannon

@uranusjr

If the extras key is missing, the default is to populate the value with all keys in optional-dependencies. If it’s explicitly given, it lists the extras that’d go into metadata.

I personally like that solution a lot.

That’s a fantastic, pragmatic proposal. What’s the next step, a PEP to update PEP 621?