Fix mocking the built-in open function for Python2

- fix import of mock module
- account for io.StringIO differences in Python 2

tests/pylorax/test_buildstamp.py

@@ -1,6 +1,6 @@
 import io
 import unittest
-from unittest.mock import patch
+from mock import patch
 
 from pylorax import buildstamp
 
@@ -16,8 +16,13 @@ class BuildStampTestCase(unittest.TestCase):
 
     def test_write_produces_file_with_expected_content(self):
         out_file = io.StringIO()
+        # io.StringIO.write accepts unicode,
+        # but on Python 2 BuildStamp.write() passes strings to this method
+        out_file.orig_write = out_file.write
+        out_file.write = lambda s: out_file.orig_write(unicode(s))
+
         with patch.object(out_file, 'close'):
-            with patch.object(buildstamp, 'open', return_value=out_file):
+            with patch('pylorax.buildstamp.open', create=True, return_value=out_file):
                 self.bstamp.write('/tmp/stamp.ini')
                 self.assertIn("[Main]\nProduct=Lorax Tests\nVersion=0.1\nBugURL=https://github.com/rhinstaller/lorax/issues\nIsFinal=True", out_file.getvalue())
                 self.assertIn("[Compose]\nLorax=devel", out_file.getvalue())