Type annotations for `nacl.bindings.crypto_aead`.

[DMRobertson]

The next module in the nacl directory I looked at pulled in from nacl.bindings, so I thought that would be the next best place to work on.

Mypy config

Due to CFFI there are lots of Anys in play here. So first, adjust the mypy config to tolerate Anys in the bindings module. The way I've done this is a bit awkward. My first attempt included nacl.bindings.crypto_aead in the existing block of overrides, together with an additional override for nacl.bindings.* which relaxed the Any checks. This doesn't work, and the mypy docs recognise this as being unfortunate.

I then tried to target the addtional override to just nacl.bindings.crypto_aead, but this failed with:

pyproject.toml: toml config file contains [[tool.mypy.overrides]]
sections with conflicting values. Module 'nacl.bindings.crypto_aead' has
two different values for 'disallow_any_expr'

Annotations

Largely copied from #692, which itself cribbed types from docstrings. There are two changes on top of that:

• I marked the constants at the top of the module as ints. They're size_ts in the corresponding C header. I'm not sure this is important (I think they get used with CFFI only?), but it drives down the number of Anys.
• The code suggested that aead was allowed to be an Optional[bytes] because we allow it to be None everywhere. I've changed the docstring and annotation to reflect this. I, erm, have no idea if passing None/NULL to the relevant functions is cryptographically meaningful or safe.

[reaperhulk]

We can probably do slightly larger PRs if that helps you out a bit. If they're < 400 lines of diff they're still reviewable 😄