Fix hacking test requirement

A recent change [1] updated the hacking version
in manila-tempest-plugin and made corresponding
fixes but unfortunately is python 2 incompatible.
We really only need hacking on master anyways,
where we no longer support python 2, so here we
make the requirement specific to python 3.

[1] https://review.opendev.org/#/c/716559

Closes-bug: #1870621

Change-Id: I9771cf31d50dbbfc0eb978ffb8f485c431cf2767
1 file changed