Gracefully degrade on v$transaction not readable

Gracefully degrade on v$transaction not readable

Fixed regression which occured due to [ticket:5755] which implemented
isolation level support for Oracle.   It has been reported that many Oracle
accounts don't actually have permission to query the ``v$transaction``
view so this feature has been altered to gracefully fallback when it fails
upon database connect, where the dialect will assume "READ COMMITTED" is
the default isolation level as was the case prior to SQLAlchemy 1.3.21.
However, explicit use of the :meth:`_engine.Connection.get_isolation_level`
method must now necessarily raise an exception, as Oracle databases with
this restriction explicitly disallow the user from reading the current
isolation level.

Fixes: #5784
Change-Id: Iefc82928744f3c944c18ae8000eb3c9e52e523bc