Replace custom-derive with macro-attr

[alexanderkjall]

Hi

I'm working on packaging this project for inclusion into Debian, and noticed that it depends on an outdated dependency custom-derive that has been superceded by macro-attr.

Would you be interested in a patch that would replace that dependency?

for reference: https://salsa.debian.org/rust-team/debcargo-conf/merge_requests/68

[jayvdb]

https://github.com/trailofbits/cargo-unmaintained reports this problem in bright red:

custom_derive (not in https://github.com/DanielKeep/rust-custom-derive)