lint/CI: Use mypy for static type checking

[jku]

We should start running mypy in CI (probably in tox "lint" environment). This would enforce type annotation use and would check types at compile time.

Possibly we only want to do this to "new code" like api/ but it might be worth it to add annotations to everything (I'm not sure how much work this is).

Current behavior:

• type annotation use is not enforced
• types are not checked at compile time

Expected behavior:

• new code without type annotations fails CI
• types are statically checked: type check failure leads to CI failure

[jku]

I'm not sure if this is a "good first issue" but getting started should not require deep knowledge of tuf.

[joshuagl]

Some good pointers in the mypy docs, such as Using mypy with an existing codebase and the import cycles section of the runtime troubles document.

[jku]

I've got a branch, currently blocked on #1378 (it happens to correctly fail a mypy check): https://github.com/jku/tuf/tree/initial-mypy-integration