Remove RFC2047 decoder (#967)
## Summary
- This was inherited from
https://github.com/PyO3/python-pkginfo-rs/blob/d719988323a0cfea86d4737116d7917f30e819e2/src/metadata.rs#LL78C2-L91C26
- ...which introduced this code here:
https://github.com/PyO3/python-pkginfo-rs/commit/9cd1d43f7c2306695f53375f50717b4f7fc0b63c
- ...with the originating issue here:
https://github.com/PyO3/maturin/issues/612
- ...and the upstream issue here:
https://github.com/staktrace/mailparse/issues/50
It seems like the goal was to support Unicode in certain header fields,
but I don't think this is necessary for us. We only use
`get_first_value` for `Requires-Python`, which has to be ASCII, doesn't
it?
In my testing, it seems like the `charset` hack can also be removed. The
tests I copied over actually work without it, which makes me a bit
skeptical.
The main benefit here is that we get to a remove a _big_ dependency
stack, including Chumsky and Stacker and psm which have limited
cross-platform support.
