Enforce type hints

[jonathanmaw]

While working on this, I have been frustrated by model diffs producing unexpected results because they have tried to compare a list of E-mail addresses with a tuple of E-mail addresses, and finding that they differ.

There are, broadly, 2 possible ways to fix this:

1. Use mypy, a static type checker for python, to check that we're using appropriate types.
2. Use type_enforced (https://github.com/connor-makowski/type_enforced) to decorate the User and Group class (and hope that it's compatible with the dataclasses decorator)

[jonathanmaw]

I don't think type_enforced will work. While trying to use it, its type enforcer complains about how dataclasses implements generating default values, i.e.

TypeError: (User.__init__): Type mismatch for typed variable `email`. Expected one of the following `[tuple[str]]` but got `<class 'dataclasses._HAS_DEFAULT_FACTORY_CLASS'>` instead.

[jonathanmaw]

Next option: See if mypy complains.

If that doesn't work: give up and just add a __post_init() method to the User and Group classes which tries to convert the emails and groups fields into tuples.