Rename build_meta:legacy to build_meta:__legacy__

changelog.d/1652.change.rst

-Added the ``build_meta:legacy`` backend, a "compatibility mode" PEP 517 backend that can be used as the default when ``build-backend`` is left unspecified in ``pyproject.toml``.
+Added the ``build_meta:__legacy__`` backend, a "compatibility mode" PEP 517 backend that can be used as the default when ``build-backend`` is left unspecified in ``pyproject.toml``.

setuptools/build_meta.py

@@ -40,7 +40,7 @@ __all__ = ['get_requires_for_build_sdist',
            'prepare_metadata_for_build_wheel',
            'build_wheel',
            'build_sdist',
-           'legacy',
+           '__legacy__',
            'SetupRequirementsError']
 
 class SetupRequirementsError(BaseException):
@@ -228,4 +228,4 @@ build_sdist = _BACKEND.build_sdist
 
 
 # The legacy backend
-legacy = _BuildMetaLegacyBackend()
+__legacy__ = _BuildMetaLegacyBackend()

setuptools/tests/test_build_meta.py

@@ -270,7 +270,7 @@ class TestBuildMetaBackend:
 
 
 class TestBuildMetaLegacyBackend(TestBuildMetaBackend):
-    backend_name = 'setuptools.build_meta:legacy'
+    backend_name = 'setuptools.build_meta:__legacy__'
 
     # build_meta_legacy-specific tests
     def test_build_sdist_relative_path_import(self, tmpdir_cwd):