Add type annotations

[jku]

We should type annotate our functions:

• Most importantly we should type annotate public methods (this should enable TUF to type check its calls)
• internal methods are useful to annotate as well (and may be necessary to enable static type checking in this repo)

This is companion to #357 (using mypy in securesystemslib): I think the process goes like this (although it's partially speculation: I've not tried this yet)

• annotate at least one module (at least the public methods)
• when public methods are annotated, this module can be enabled in TUF mypy config
• when one whole module is annotated, we can do Use mypy to statically check sources #357 and start type checking here

Repeat steps for each module

Using mypy with an existing codebase may be useful: we may want to try tools to automatically generate type annotations: they might work well for this sort of code

[sechkova]

mypy is now included in the CI run with #361.
Also the util and storage modules are type annotated with #361 and #362.

[jku]

The other modules used in TUF (metadata api) are:

exceptions
formats
keys
signer

So I guess those should be next in line (but I think we could also enable checks in TUF module-by-module if there are trickier modules that can't be annotated in the next weeks)

[jku]

Also, I think we need to add a securesystemslib/py.typed marker file in the repo: https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-type-hints-for-third-party-library
Otherwise TUF can't use the SSLib annotations (and we definitely want that because we want to find things like theupdateframework/python-tuf#1442)

[jku]

Also, I think we need to add a securesystemslib/py.typed

3 years later this is still true 😂