Add build_meta_legacy backend

This is part of the solution to GH #1642, it is a
backwards-compatibility backend that can be used as a default PEP 517
backend for projects that use setuptools but haven't opted in to
build_meta.

setuptools/build_meta_legacy.py

@@ -0,0 +1,44 @@
+"""Compatibility backend for setuptools
+
+This is a version of setuptools.build_meta that endeavors to maintain backwards
+compatibility with pre-PEP 517 modes of invocation. It exists as a temporary
+bridge between the old packaging mechanism and the new packaging mechanism,
+and will eventually be removed.
+"""
+
+import sys
+
+from setuptools.build_meta import _BuildMetaBackend
+from setuptools.build_meta import SetupRequirementsError
+
+
+__all__ = ['get_requires_for_build_sdist',
+           'get_requires_for_build_wheel',
+           'prepare_metadata_for_build_wheel',
+           'build_wheel',
+           'build_sdist',
+           'SetupRequirementsError']
+
+
+class _BuildMetaLegacyBackend(_BuildMetaBackend):
+    def run_setup(self, setup_script='setup.py'):
+        # In order to maintain compatibility with scripts assuming that
+        # the setup.py script is in a directory on the PYTHONPATH, inject
+        # '' into sys.path. (pypa/setuptools#1642)
+        sys_path = list(sys.path)       # Save the old path
+        if '' not in sys.path:
+            sys.path.insert(0, '')
+
+        super(_BuildMetaLegacyBackend,
+              self).run_setup(setup_script=setup_script)
+
+        sys.path = sys_path             # Restore the old path
+
+
+_BACKEND = _BuildMetaLegacyBackend()
+
+get_requires_for_build_wheel = _BACKEND.get_requires_for_build_wheel
+get_requires_for_build_sdist = _BACKEND.get_requires_for_build_sdist
+prepare_metadata_for_build_wheel = _BACKEND.prepare_metadata_for_build_wheel
+build_wheel = _BACKEND.build_wheel
+build_sdist = _BACKEND.build_sdist

setuptools/tests/test_build_meta.py

@@ -251,7 +251,6 @@ def test_build_sdist_relative_path_import(self, build_backend, tmpdir_cwd):
             build_backend.build_sdist("temp")
 
 
-@pytest.mark.xfail
 class TestBuildMetaLegacyBackend(TestBuildMetaBackend):
     backend_name = 'setuptools.build_meta_legacy'