Update annotations to python 3.9

[jku]

#886 is merged so we could start using "new" annotations in the code base. Fixing this requires no securesystemslib knowledge but understanding Python annotation basics is required.

Python 3.9 allows annotations using standard library containers (so e.g. dict instead of typing.Dict). In practice this should work:

• bump requires-python in pyproject.toml
• run ruff check --fix --unsafe-fixes securesystemslib tests
• review that changes made by ruff are sensible
• make sure tox -e lint and tox -e test pass