neutron-tempest-plugin-designate-scenario fails on stable branches with "SyntaxError: invalid syntax" installing dnspython

Most probably a follow-up of https://review.opendev.org/#/c/704084/ dropping py2.7 testing, stable branches reviews now fail on neutron-tempest-plugin-designate-scenario job with failure like:
2020-02-20 08:05:43.987705 | controller | 2020-02-20 08:05:43.987 | Collecting dnspython3!=1.13.0,!=1.14.0,>=1.12.0 (from designate-tempest-plugin==0.7.1.dev1)
2020-02-20 08:05:44.204759 | controller | 2020-02-20 08:05:44.204 | Downloading http://mirror.ord.rax.opendev.org/pypifiles/packages/f0/bb/f41cbc8eaa807afb9d44418f092aa3e4acf0e4f42b439c49824348f1f45c/dnspython3-1.15.0.zip
2020-02-20 08:05:44.734839 | controller | 2020-02-20 08:05:44.734 | Complete output from command python setup.py egg_info:
2020-02-20 08:05:44.734956 | controller | 2020-02-20 08:05:44.734 | Traceback (most recent call last):
2020-02-20 08:05:44.735018 | controller | 2020-02-20 08:05:44.734 | File "<string>", line 1, in <module>
2020-02-20 08:05:44.735078 | controller | 2020-02-20 08:05:44.735 | File "/tmp/pip-build-gSLP9k/dnspython3/setup.py", line 25
2020-02-20 08:05:44.735165 | controller | 2020-02-20 08:05:44.735 | """+"="*78, file=sys.stdout)
2020-02-20 08:05:44.735231 | controller | 2020-02-20 08:05:44.735 | ^
2020-02-20 08:05:44.735290 | controller | 2020-02-20 08:05:44.735 | SyntaxError: invalid syntax

Sample reviews:
https://review.opendev.org/#/c/708576/ (queens)
https://review.opendev.org/#/c/705421/ (rocky)
https://review.opendev.org/#/c/708488/ (stein)

As "[ussuri][goal] Drop python 2.7 support and testing" is the only new commit since tag 0.7.0 in designate-tempest-plugin, we can try to use this tag for our py2 stable branches jobs