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.

pyproject-rpm-macros.spec

@@ -6,7 +6,7 @@ License:        MIT
 
 # Keep the version at zero and increment only release
 Version:        0
-Release:        11%{?dist}
+Release:        12%{?dist}
 
 Source0:        macros.pyproject
 Source1:        pyproject_buildrequires.py
@@ -87,6 +87,10 @@ install -m 644 pyproject_buildrequires.py %{buildroot}%{_rpmconfigdir}/redhat/
 %license LICENSE
 
 %changelog
+* Wed Feb 05 2020 Miro Hrončok <mhroncok@redhat.com> - 0-12
+- Fallback to setuptools.build_meta:__legacy__ backend instead of setuptools.build_meta
+- Properly handle backends with colon
+
 * Thu Jan 30 2020 Fedora Release Engineering <releng@fedoraproject.org> - 0-11
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_32_Mass_Rebuild
 

pyproject_buildrequires.py

@@ -139,18 +139,29 @@ def get_backend(requirements):
 
     backend_name = buildsystem_data.get('build-backend')
     if not backend_name:
+        # 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 [following] backend).
+        backend_name = 'setuptools.build_meta:__legacy__'
+
         requirements.add('setuptools >= 40.8', source='default build backend')
         requirements.add('wheel', source='default build backend')
 
-        backend_name = 'setuptools.build_meta'
-
     requirements.check(source='build backend')
 
     backend_path = buildsystem_data.get('backend-path')
     if backend_path:
         sys.path.insert(0, backend_path)
 
-    return importlib.import_module(backend_name)
+    module_name, _, object_name = backend_name.partition(":")
+    backend_module = importlib.import_module(module_name)
+
+    if object_name:
+        return getattr(backend_module, object_name)
+
+    return backend_module
 
 
 def generate_build_requirements(backend, requirements):