Upstream konstin/pep508_rs#17 (#6200)
Upstream https://github.com/konstin/pep508_rs/pull/17
> This removes the `derivative` dependency which [seems to be
unmaintained](https://github.com/mcarton/rust-derivative/issues/117) and
depends on old versions of some crates, especially `syn`.
>
> I could also replace it with another crate like `educe` or
`derive-where` but the implementation seems simple enough.
