This is PEP8 convention for a const variable. Also, make the type
Mapping, which doesn't have mutation methods. This means mypy will warn
us if someone tries to change this.
This patches takes the options work to it's logical conclusion: A single
flat dictionary of OptionKey: UserOptions. This allows us to simplify a
large number of cases, as we don't need to check if an option is in this
dict or that one (or any of 5 or 6, actually).