Replace custom URL-encoding method with quote_plus

Replace custom URL-encoding method with quote_plus

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_plus``, 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: I90b7a9f4dfdb719082b4b178ad4e009a8531a18e