In Python 3, the attribute BaseException.message doesn't exist.
$ python3
>>> from sphinx.errors import ExtensionError
>>> e = ExtensionError('foo')
>>> repr(e)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "sphinx/sphinx/errors.py", line 65, in __repr__
return '%s(%r)' % (self.__class__.__name__, self.message)
AttributeError: 'ExtensionError' object has no attribute 'message'
