Add support for python 3.8
Add support for python 3.8

Introduce support of python 3.8 [1] and move tox and jobs to py38.

jsonschema 3.2.0 [2] support python 3.8
pyOpenSSL 19.1.0 [3] support python 3.8

[1] https://governance.openstack.org/tc/reference/runtimes/victoria.html#python-runtimes-for-victoria
[2] https://github.com/Julian/jsonschema/pull/627
[3] https://pypi.org/project/pyOpenSSL/19.1.0/

Change-Id: I8f561325780a95173501f42e10b6fce617bb17f4

openstack/python-troveclient,openstack/python-troveclient