[3.11] gh-94751: Install, import and run the test C++ extension (MVP) (GH-94754)

[miss-islington]

This is a quick-and-dirty way to run the C++ tests.
It can definitely be improved in the future, but it should fail when things go wrong.

• Run test functions on import (yes, this can definitely be improved)
• Fudge setuptools metadata (name & version) to make the extension installable
• Install and import the extension in test_cppext
  (cherry picked from commit ec5db53)

Co-authored-by: Petr Viktorin encukou@gmail.com

• Issue: test_cppext does not import/run the C++ extension #94751

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🤖 New build scheduled with the buildbot fleet by @encukou for commit 1ab009d 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

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@encukou: Status check is done, and it's a failure ❌ .

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