fix issue413: exceptions with unicode attributes are now printed

correctly also on python2 and with pytest-xdist runs. (the fix
requires py-1.4.20)

setup.py

@@ -17,7 +17,7 @@ classifiers=['Development Status :: 6 - Mature',
 
 long_description = open("README.rst").read()
 def main():
-    install_requires = ["py>=1.4.19"]
+    install_requires = ["py>=1.4.20.dev2"]
     if sys.version_info < (2,7):
         install_requires.append("argparse")
     if sys.platform == "win32":