specify encoding when opening file in test_history

fixes error when running in ASCII locale

IPython/core/tests/test_history.py

@@ -93,7 +93,7 b' def test_history():'
             # Cross testing: check that magic %save can get previous session.
             testfilename = os.path.realpath(os.path.join(tmpdir, "test.py"))
             ip.magic("save " + testfilename + " ~1/1-3")
-            with py3compat.open(testfilename) as testfile:
+            with py3compat.open(testfilename, encoding='utf-8') as testfile:
                 nt.assert_equal(testfile.read(),
                                         u"# coding: utf-8\n" + u"\n".join(hist))