include TableClause.schema in cache key

include TableClause.schema in cache key

Fixed issue where use of the :func:`_sql.table` construct, passing a string
for the :paramref:`_sql.table.schema` parameter, would fail to take the
"schema" string into account when producing a cache key, thus leading to
caching collisions if multiple, same-named :func:`_sql.table` constructs
with different schemas were used.

Fixes: #8441
Change-Id: Ic4b55b3e8ec53b4c88ba112691bdf60ea1d4c448