Bug#1124698: pybuild-plugin-pyproject: Install a clean extraction of the wheel (not the test playground)

[Stefano Rivera]

Package: dh-python
Version: 7.20251231
Severity: normal

We've seen instances of tests leaving junk in the tree that we install. 
That's not ideal. We should run the tests in a separate extraction of 
the wheel, even if this is a bit slower.

Stefano