Type hints napalm.base and napalm.nxos

[Kircheneer]

I have now gone ahead and put type hints in for napalm.base and napalm.nxos. I am aware that this is a lot of change for a single pull request, as a good amount of code to be executed at runtime has been added as well (lots of assert isinstance(...)). I wanted to make sure that all of the typing errors are gone before I submitted the PR because only then I can know that the whole type hint construct is more or less well defined. If I was still missing bits, other - already annotated - bits might not type check correctly. While I have made an effort as to not cause any regressions through these runtime behavior changes I still think that this PR needs a rather thorough review due to this. I'm more than happy to have a discussion here.

I have also added Mypy to the GitHub Actions workflow file.

There is a final discussio point about a type error which I have currently silenced (mypy ran without the # type: ignore in this snippet).

$ mypy -p napalm
napalm\base\helpers.py:304: error: Incompatible default for argument "default" (default has type "str", argument has type "R")
Found 1 error in 1 file (checked 58 source files)

I have put generic type variables in for the convert function as follows:

def convert(to: Callable[[T], R], who: Optional[T], default: R = "") -> R:  # type: ignore
    ...

Mypy complains about this because "" is not of type R. I held off on changing this due to the ~200 occurences of this function within the various drivers as I did not want to change anything in any drivers but the NXOS and the base one. My proposed solution would be to go through and make sure a default is passed for every call.

[mirceaulinic]

Nice work. I think I like the approach.

Any thoughts on this @ktbyers?