Replace custom URL-encoding method with quote

Replace custom URL-encoding method with quote

Fixed URL-encoding of the username and password components of
:class:`.engine.URL` objects when converting them to string using the
:meth:`_engine.URL.render_as_string` method, by using Python standard
library ``urllib.parse.quote`` while allowing for plus signs and spaces to
remain unchanged as supported by SQLAlchemy's non-standard URL parsing,
rather than the legacy home-grown routine from many years ago. Pull request
courtesy of Xavier NUNN.

Fixes: #10662
Closes: #10726
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10726
Pull-request-sha: 82219041b8f73d8c932cc40e87c002b3b853e02e
Change-Id: Iedca4929579d4d26ef8cce083252dcd1e476286b