Use Unicode literal.

topydo/lib/TodoBase.py

@@ -20,7 +20,7 @@ This module contains the class that represents a single todo item.
 
 from datetime import date
 import re
-from six import python_2_unicode_compatible
+from six import python_2_unicode_compatible, u
 
 from topydo.lib.TodoParser import parse_line
 from topydo.lib.Utils import is_valid_priority
@@ -213,7 +213,7 @@ class TodoBase(object):
         self.src = re.sub(
             r'^(x \d{4}-\d{2}-\d{2} |\([A-Z]\) )?(\d{4}-\d{2}-\d{2} )?(.*)$',
             lambda m: \
-            "{}{} {}".format(m.group(1) or '', p_date.isoformat(), m.group(3)),
+            u("{}{} {}").format(m.group(1) or '', p_date.isoformat(), m.group(3)),
             self.src)
 
     def creation_date(self):