Stop falling back to setup.py install by default

[pradyunsg]

Toward #8368

[sbidoul]

@pradyunsg before doing this don't we need to disentangle --no-binary first ? IOW finish #9422, which I still hope to find the time to do someday.

[pradyunsg]

Yes we do.

I opened this as a draft, to get a better idea of how complicated this removal would be in terms of the test suite (by running CI on this change) -- given that the code change is fairly "cheap". :)

[pradyunsg]

FAILED tests/functional/test_install_index.py::test_find_links_no_doctype - a...
FAILED tests/functional/test_install.py::test_install_wheel_broken - Assertio...
FAILED tests/functional/test_install.py::test_cleanup_after_failed_wheel - As...
FAILED tests/functional/test_install.py::test_install_builds_wheels - Asserti...
FAILED tests/functional/test_install.py::test_install_no_binary_disables_building_wheels

Not too bad. :)