Add type hints

[jannikluhn]

We should add type hints and check them in CI.

[Bhargavasomu]

I will take this up once #17 is merged.

[jannikluhn]

@Bhargavasomu Note that most of this should be done already in #22.

[hwwhww]

@Bhargavasomu would you like to take it + enable mypy in tox?

[Bhargavasomu]

@hwwhww I will take this up.

[jannikluhn]

There's not a lot of documentation and it's unstable, but if we want to have type hints for Serializable.__init__ (which I think we should), we probably have to write a mypy plugin.

[pacrob]

There are a lot of types throughout the lib, but it doesn't appear that mypy checks were ever run as part of CI. With the incoming template updates, mypy is being added as part of the pre-commit checks, but with the ssz folder excluded. As files are typed, they can be added to be checked. See pre-commit docs