Potential Bug: Use of mutable default arguments in python

[elayeek]

This is a niche issue but this is a common pitfall in Python that causes unexpected behavior.

Python evaluates functions on module load and generates mutable default argument objects once, then persist them whenever the function is called.

https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

[elayeek]

Minor PR to fix it (I need to run tests and linter again)

https://github.com/TabbycatDebate/tabbycat/pull/2482