Fix test on py27

Author: nicoddemus

testing/test_terminal.py

@@ -5,8 +5,6 @@
 import collections
 import sys
 
-import io
-
 import pluggy
 import _pytest._code
 import py
@@ -998,7 +996,7 @@ def write_and_check(s, expected):
 
 def test_write_updates_line_chars_count(testdir):
     config = testdir.parseconfig()
-    f = io.StringIO()
+    f = py.io.TextIO()
     tr = TerminalReporter(config, file=f)
     assert tr._current_line_chars == 0
     tr.write('foo')