six.reraise can be converted into "raise new_exc from original_exc"
introduced in python3.
In case of horizon.exceptions.handle(), six.reraise was used to
raise the original exception again, so we can convert it into "raise".
six is removed from requirements.txt.
Note that we need to keep six in lower-constraints.txt
as dependent libraries still use six.
Change-Id: I0672a516083727c245f1d4fa5b10eed9cd81a726