Use assertEqual for Python 3.11 compatibility

[hugovk]

assertEquals has been deprecated since Python 3.2 and is removed in 3.11. Use assertEqual instead.

https://docs.python.org/3.11/whatsnew/3.11.html#removed

[hugovk]

Update:

This was retained in Python 3.11 (released October 2022), but has already been removed in Python 3.12 (set for release October 2023):

• https://docs.python.org/3/whatsnew/3.11.html#pending-removal-in-python-3-12
• https://docs.python.org/3.12/whatsnew/3.12.html#removed

I still recommend merging this fix. However, I also see there's been no commits for ~six years, so closing this year-old PR, but happy to re-open if there's interest 👍