Pass URL object, not the string, to on_connect_url

Pass URL object, not the string, to on_connect_url

The fix for pysqlcipher released in version 1.4.3 :ticket:`5848` was
unfortunately non-working, in that the new ``on_connect_url`` hook was
erroneously not receiving a ``URL`` object under normal usage of
:func:`_sa.create_engine` and instead received a string that was unhandled;
the test suite failed to fully set up the actual conditions under which
this hook is called. This has been fixed.

Fixes: #6586
Change-Id: I3bf738daec35877a10fdad740f08dca9e7420829