[internal / maintenance / tests] TO-DO list

[alessio-locatelli]

• Enable check_untyped_defs = true (see Update Mypy config and type annotations #220)

• Add a benchmark and stress test per cache backend, and with cache disabled. Additionally, for the "memory" cache backend I would like to know the memory usage.

• Investigate https://github.com/orgs/requests-cache/discussions/903