Allow url.password to be an object

Allow url.password to be an object

The "password" attribute of the :class:`.url.URL` object can now be
any user-defined or user-subclassed string object that responds to the
Python ``str()`` builtin.   The object passed will be maintained as the
datamember :attr:`.url.URL.password_original` and will be consulted
when the :attr:`.url.URL.password` attribute is read to produce the
string value.

Change-Id: I91d101c3b10e135ae7e4de60a5104b51776db84f
Fixes: #4089