Add type hints

[fkromer]

The lack of Python type hints makes things more complicated/in-convenient than necessary:

• IDE builtin support for viewing reference documentation cannot be used. E.g. viewing reference documentation in PyCharm.
  • Developing the pytm code base is harder.
  • Developing threat models is harder.

[happypeepo]

Hi, I’d like to work on this issue if it’s still open. Could I take this up?