Add hint for DevPi indexes without /+simple

[zanieb]

Summary

Without the /+simple suffix, we'll report no versions on devpi indexes, e.g.:

• Issue Fetching Packages from Devpi Server Using uv and --index-url #10567
• uv fails to install package (binary wheel) from internal index while pip succeeds #11135

We may be able to sniff it is a devpi index with <h1><a href="https://<host>/">devpi</a></h1> and hint the fix to the user.

This is an alternative to adding support as described in #4907

Example

  × No solution found when resolving dependencies:
  ╰─▶ Because there are no versions of package and you require package, we can conclude that the requirements
      are unsatisfiable.

hint: It looks like you are using a DevPi index but did not suffix the index URL with `/+simple` which is
      required for uv to use the index.

[charliermarsh]

Good idea.

[zanieb]

Seems kind of painful to thread through, but might not be that bad.

[charliermarsh]

It's probably hard to thread through the hint, but could we fail entirely when we read from this URL, rather than making it a hint?

[zanieb]

I'm not sure how much I trust the heuristic I shared, we might need something clearer if we're going to fail hard.

[charliermarsh]

We can probably use (or misuse) IndexCapabilities for this. It's how we show the 401 errors, etc.

[charliermarsh]

It... turns out that devpi sniffs the user agent to determine if a request is coming from something that "looks like" an installer: https://github.com/devpi/devpi/blob/18a53a169865f770defa2932d69b75dbf4f869a4/server/devpi_server/views.py#L75.

So if you have devpi running locally per the quickstart, this works:

curl http://testuser:123@localhost:3141/testuser/dev/flask -H "Accept: application/vnd.pypi.simple.v1+json, application/vnd.pypi.simple.v1+html;q=0.2, text/html;q=0.01" --user-agent "uv/1.0"

But this does not:

curl http://testuser:123@localhost:3141/testuser/dev/flask -H "Accept: application/vnd.pypi.simple.v1+json, application/vnd.pypi.simple.v1+html;q=0.2, text/html;q=0.01"

uv was added in July. So newer versions should "just work", but older versions do not work automatically.