Tighten password security by removing `URL.__str__`

Tighten password security by removing `URL.__str__`

For improved security, the :class:`_url.URL` object will now use password
obfuscation by default when ``str(url)`` is called. To stringify a URL with
cleartext password, the :meth:`_url.URL.render_as_string` may be used,
passing the :paramref:`_url.URL.render_as_string.hide_password` parameter
as ``False``. Thanks to our contributors for this pull request.

Fixes: #8567
Closes: #8563
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8563
Pull-request-sha: d1f1127f753849eb70b8d6cc64badf34e1b9219b

Change-Id: If756c8073ff99ac83876d9833c8fe1d7c76211f9