trust empty requires dist with modern metadata

[dimbleby]

when modern metadata says that it has no requires-dist, we can trust it

I bet it doesn't often matter:

• https://github.com/pypi/warehouse/pull/13606 was only just released
• as of today presumably more or less no-one (poetry included) is yet publishing metadata versioned at 2.2 or higher
• and probably nearly all newly published packages also publish wheels anyway.

still, when it happens it happens, and avoiding unnecessary builds is a good win

[dimbleby]

prior to their 1.10, the pkginfo parsing of files with metadata 2.3 failed to return any useful values.

this is more or less a proof that no-one was declaring metadata version 2.3: because if they did then things would surely go wrong.

However I see that maturin has just merged a change such that their backend will start declaring metadata version 2.3, and it looks as though they are about to cut a 1.5.0 release, and so fairly soon there will likely be a fresh supply of metadata-2.3 packages. Once that happens I would not be surprised to see a little flurry of issues for which the answer will be "upgrade pkginfo in your poetry installation".

I've bumped the pkginfo requirement in this MR to 1.10, and made one of the test fixtures use metadata version 2.3.

[dimbleby]

I see that #9130 rediscovered some of the above

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