Add minimal test replicating the issue. Use backports.unittest_mock==1.2 and tests pass. Ref pytest-dev/pytest#2419.

Author: jaraco

setup.py

@@ -0,0 +1,6 @@
+import setuptools
+
+setuptools.setup(
+	name='testproj',
+	version='1.0',
+)

test_issue.py

@@ -0,0 +1,21 @@
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def try_backports_import_nowrite(monkeypatch):
+	"""
+	Simulate what happens in a setuptools sandbox when
+	it attempts to prevent writing outside of the sandbox.
+	"""
+	def dont_mkdir(*args, **kwargs):
+		raise RuntimeError("Don't make dirs")
+
+	monkeypatch.setattr('os.mkdir', dont_mkdir)
+
+	try:
+		__import__('backports.does_not_exist')
+	except ImportError:
+		pass
+
+def test_sunshine_and_roses():
+	pass

tox.ini

@@ -0,0 +1,7 @@
+[testenv]
+deps =
+	pytest
+	backports.unittest_mock==1.3
+
+usedevelop = True
+commands = py.test {posargs}