Add RFC 9457 Problem Details support for Simple API errors

[luisgcoding]

This PR implements RFC9457 support for PyPI's Simple API error responses, replacing plain text error messages with standardized, machine-readable JSON error responses.

This is part of a Pre-PEP work discussed here.

Current PyPi behaviour when executing command curl -i http://localhost:80/simple/nonexistent-package/ :

Now with the change proposed here:

[warsaw]

When I was looking at the test, it seemed odd that extensions were passed in as a dictionary, but got flattened in the JSON result. I was put off by the different shape, but now I see why; it's because ProblemDetails is a dataclass, which doesn't have a way to specify **kws. I think that's okay, except that the way it's written, it's possible for keys in extensions to override the required arguments, e.g.

pd = ProblemDetails(400, "Bad request", extensions={"status": 200})

Maybe that's not terrible, but maybe it makes sense to do some extra checking before the result.update() on line 66?

I also wonder is whether extensions should be a kw_only field?

[luisgcoding]

I added validation and made the field kw_only here.

[warsaw]

My reading of about:blank says it can be implicit but doesn't need to be. Should we default to explicit?

Also should we support the SHOULD "the title SHOULD be the same as the recommended HTTP status phrase for that code"?

[luisgcoding]

I added the SHOULD here in method problem_details_from_exception. I think is better to be implicit but happy to change it otherwise.

[warsaw]

Could be:

    if detail is None:
        problem_detail = getattr(...
    else:
        problem_detail = detail

or a ternary expression, but I don't think that would make things more readable.

[luisgcoding]

Fixed here

[warsaw]

There was some question on DPO whether we wanted to limit this response to only requests that explicitly accept application/problem+json, although now that I'm looking I don't see accepts_problem_json() called anywhere.

I'm unsure so maybe @woodruffw can chime in?

[luisgcoding]

I removed the accepts_problem_json for the moment. We can add it back if we decide to use it for backwards compatibility.