Pin stable/2023.2 constraint in tox to release 37.0.0

We are going to release Tempest new tag 37.0.0 to declare
the end of support for stable/yoga. stable/2023.2 constraint
is latest stable constraints tested with current tempest hash
which is going to be used for 37.0.0 release.

Further information:
 https://docs.openstack.org/tempest/latest/requirement_upper_constraint_for_tempest.html

Change-Id: I0b4863c219b71b75f665e7a7599063124c34b2de
2 files changed