[py] Fix encoding of basic auth header when using Python 3

Fixes SeleniumHQ#3622

Author: davehunt

py/selenium/webdriver/remote/remote_connection.py

@@ -476,8 +476,8 @@ def _request(self, method, url, body=None):
             request.add_header('Content-Type', 'application/json;charset=UTF-8')
 
             if parsed_url.username:
-                base64string = base64.b64encode('%s:%s' % (parsed_url.username, parsed_url.password))
-                request.add_header("Authorization", "Basic %s" % base64string)
+                base64string = base64.b64encode('{0.username}:{0.password}'.format(parsed_url).encode())
+                request.add_header('Authorization', 'Basic {}'.format(base64string).decode())
 
             if password_manager:
                 opener = url_request.build_opener(url_request.HTTPRedirectHandler(),