unvendor `multipart`

[glyph]

the underlying issue has been addressed so we shouldn't need to carry the patch any more

[cjwatson]

It's possibly worth noting that there are still problems to sort out at the downstream level, such as https://bugs.debian.org/1085728 (I wouldn't be surprised if other distributions had similar problems). Though I suppose we could always re-vendor as a temporary distro patch if we had to.

[glyph]

It's possibly worth noting that there are still problems to sort out at the downstream

If the distro wants to mess up their deps, they are welcome to mess up our usage of the deps as well ;)

But I am OK with slow-rolling this — we should just do it eventually.