fix(mfa): Add types to allauth.mfa flows

[akx]

This PR adds a bunch of types to allauth.mfa's internals.

Our app needs to set up MFA with a slightly custom flow, so we can't use the forms or headless features, so I needed to delve a bit into how things work under the hood, and figured it'd be useful if types were documented technically too, for IDEs etc.

Feels like TOTP and RecoveryCodes should have a superclass, but that's beyond the scope of this PR...

[coveralls]

coverage: 95.749% (+0.001%) from 95.748%
when pulling c2465b2 on akx:mfa-types
into 46e4b51 on pennersr:main.

[pennersr]

Meanwhile, I have added mypy checking to the build, as adding type hints without enforcing them easily leads to errors. This did cause conflicts with your changes though. Can you please rebase? Thanks.

[akx]

Can you please rebase? Thanks.

Done, let's see if it passes! 🤞

adding type hints without enforcing them easily leads to errors.

Tangentially: I know we've talked about Ruff before and I know how you feel about it, but it was pretty useful here, as the ANN series of rules can narrow down places that are missing annotations, and a human can then figure them out.

[pennersr]

See #3905 -- some of your changes are not Python 3.7 compatible, that will be addressed by #3905.

[akx]

@pennersr On that note, what's your stance on using from __future__ import annotations and futuristic annotations (e.g. list[Foo] | None)? I don't think anything in allauth is expecting to introspect annotations (where unevaluated futured annotations could be an issue).