Fix test since pip 10 changed output

tests/languages/helpers_test.py

@@ -1,7 +1,13 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
+import sys
+
+import pytest
+
 from pre_commit.languages import helpers
+from pre_commit.prefix import Prefix
+from pre_commit.util import CalledProcessError
 
 
 def test_basic_get_default_version():
@@ -10,3 +16,15 @@ def test_basic_get_default_version():
 
 def test_basic_healthy():
     assert helpers.basic_healthy(None, None) is True
+
+
+def test_failed_setup_command_does_not_unicode_error():
+    script = (
+        'import sys\n'
+        "getattr(sys.stderr, 'buffer', sys.stderr).write(b'\\x81\\xfe')\n"
+        'exit(1)\n'
+    )
+
+    # an assertion that this does not raise `UnicodeError`
+    with pytest.raises(CalledProcessError):
+        helpers.run_setup_cmd(Prefix('.'), (sys.executable, '-c', script))