Avoid unittest deprecated aliases

Summary:
References:

• https://docs.python.org/3.10/library/unittest.html#deprecated-aliases

Ref T2123

Test Plan: (cd _tests && python3 -m unittest modules/test_prometheus.py)

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2123

Differential Revision: https://devcentral.nasqueron.org/D3661