Use the logic already vetted for determining ascii context.

setuptools/tests/test_easy_install.py

@@ -170,7 +170,7 @@ class TestEasyInstallTest:
         sdist_zip.close()
         return str(sdist)
 
-    @pytest.mark.xfail(os.environ.get('LANG') == 'C',
+    @pytest.mark.xfail(setuptools.tests.is_ascii,
         reason="https://github.com/pypa/setuptools/issues/706")
     def test_unicode_filename_in_sdist(self, sdist_unicode, tmpdir, monkeypatch):
         """