feat(pypi): support PEP691 JSON API #282

Thanks for the project!

During setup I've noticed that the pypi handler does not yet support the PEP691 JSON API. Modern package managers like uv rely heavily upon the there provided metadata.

On a quick glance I think the reason is that the handler is requesting the text/html version while a package manager supporting the standard, would send Accept: application/vnd.pypi.simple.v1+json which must be passed to pypi for content negotiation. Currently. this blocks usage with uv and probably other package managers which us the metadata.

Reproduce:

#pyproject.toml
[project]
name = "test"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
authors = []
requires-python = ">=3.13"
dependencies = ["ruff>=0.16.0"]


[tool.uv]
package = false
exclude-newer = "7 days"  # enable / disable to trigger-   
no-build = true

[[tool.uv.index]]
url = "http://localhost:18080/pypi/simple/"
default = true

uv lock

or even simpler:

# public index
curl \                                                                                                                                                          
  -H 'Accept: application/vnd.pypi.simple.v1+json' \
  https://pypi.org/simple/ruff/ | jq

# a local proxy instance
curl \                                                                                                                                                          
  -H 'Accept: application/vnd.pypi.simple.v1+json' \
  http://localhost:18080/pypi/simple/ruff/

I would be willing to work on this because I need it, but that would probably be end of next week earliest.