Add pure wheel detection, and failing with an error in this case

test/test_pure_wheel.py

@@ -0,0 +1,36 @@
+import subprocess
+import pytest
+from test import test_projects
+from . import utils
+
+pure_python_project = test_projects.TestProject()
+pure_python_project.files['setup.py'] = '''
+from setuptools import Extension, setup
+
+setup(
+    name="spam",
+    py_modules=['spam'],
+    version="0.1.0",
+)
+'''
+
+pure_python_project.files['spam.py'] = '''
+def a_function():
+    pass
+'''
+
+
+def test(tmp_path, capfd):
+    # this test checks that if a pure wheel is generated, the build should
+    # fail.
+    project_dir = tmp_path / 'project'
+    pure_python_project.generate(project_dir)
+
+    with pytest.raises(subprocess.CalledProcessError):
+        actual_wheels = utils.cibuildwheel_run(project_dir)
+        print('produced wheels:', actual_wheels)
+
+    captured = capfd.readouterr()
+    print('out', captured.out)
+    print('err', captured.err)
+    assert "Build failed because a pure Python wheel was generated" in captured.err