CLN: make DTA/TDA check clearer
@jbrockmendel @jreback Could you explain why this change is needed, please?
I think this change is part of the cause of https://github.com/pandas-dev/pandas/issues/23735 and  https://github.com/pandas-dev/pandas/issues/34531 > Could you explain why this change is needed, please?

an isinstance check is much clearer than a hasattr check.  It isnt clear to me how this would affect performance anywhere unless we were doing this hasattr check somewhere else to get to a fastpath