gh-92584: test_cppext uses setuptools

[vstinner]

Rewrite test_cppext to run in a virtual environment and to build the
C++ extension with setuptools rather than distutils.

[vstinner]

If this approach works and is accepted, we can use the same for test_peg_generator.

[FFY00]

What happens if CPython for some reason breaks setuptools?

[vstinner]

What happens if CPython for some reason breaks setuptools?

The test fails which is a good thing :-)

[vstinner]

Let's try and see how it goes :-)

[vstinner]

Let's try and see how it goes :-)

It fails when _ctypes is not available: #92820

@FFY00: An example of failing setuptools :-)