Handle backends with colon, fallback to setuptools.build_meta:__legacy__
Falling back to setuptools.build_meta:__legacy__ is the standard behavior,
not setuptools.build_meta. See PEP 517:
https://www.python.org/dev/peps/pep-0517/
> If the pyproject.toml file is absent, or the build-backend key is missing,
> the source tree is not using this specification, and tools should revert
> to the legacy behaviour of running setup.py (either directly, or by
> implicitly invoking the setuptools.build_meta:__legacy__ backend).
Falling back to setuptools.build_meta had very similar results so far.,
but the behavior might change in the feature.
While working on this, I have uncovered a problem in our code.
It was not able to handle backends with ":". Looking at PEP 517 again:
> build-backend is a string naming a Python object that will be used to
> perform the build. This is formatted following the same module:object syntax
> as a setuptools entry point. For instance, if the string is "flit.api:main",
> this object would be looked up by executing the equivalent of:
>
> import flit.api
> backend = flit.api.main
>
> It's also legal to leave out the :object part, e.g.
>
> build-backend = "flit.api"
>
> which acts like:
>
> import flit.api
> backend = flit.api
We now handle such cases properly. Witch the change of the default backend,
we also test a backend with colon in our tests.