tests: add syntax warnings on Python 3.8 in python-zstandard

Authored by indygreg.

Description

tests: add syntax warnings on Python 3.8 in python-zstandard

These are warnings from upstream test code. Let's just acknowledge them
for now. These should go away in a future python-zstandard release :)

Differential Revision: https://phab.mercurial-scm.org/D5846

Details