uv-fs: transparently support reading UTF-16 files
This PR tweaks uv to support reading `requirements.txt` regardless of
whether it is encoded as UTF-8 or UTF-16. This is particularly relevant
on Windows where `uv pip freeze > requirements.txt` will likely write a
UTF-16 encoded `requirements.txt` file.
There is some discussion on #1666 where it's suggested that perhaps
we should explicitly not support this. I didn't see that until I
had already put this PR together, but even so, I think it's worth
considering this. UTF-16 is predominant on Windows. It is very easy
to produce a UTF-16 encoded file. Moreover, there is an easy and well
specified way to recognize and transcode UTF-16 encoded data to UTF-8.
I think the downside of this is that it could encourage the use UTF-16
encoded `requirements.txt` files *in addition* to UTF-8 encoded
files, and it would probably be nice to converge and standardize on
one encoding. One possible alternative to this PR is that we provide
a better error message. Another alternative is to ensure that a
`-o/--output` flag exists for all commands (neither `uv pip freeze` nor
`pip freeze` have such a flag) so that users can always write output
to a file without relying on their environment's piping behavior.
(Although this last alternative seems a little sad to me.)
It's also worth noting the [PEP-0508] doesn't seem to mention file
encoding at all. So I think from a "do the standards allow this"
perspective, this change is OK.
Finally, `pip` itself seems to work with UTF-16 encoded
`requirements.txt` files.
I think I personally overall lean towards supporting UTF-16 for
`requirements.txt` files. In part because I think it smoothes out the
UX a little bit, in part because there is no obvious specification
(that I'm aware of) that mandates that these files are UTF-8, and
finally in part because `pip` supports it too.
Fixes #1666, Fixes #2276
[PEP-0508]: https://peps.python.org/pep-0508/
