nsxt: Typing fixes for Python 3.6 compatibility.

[ivucica]

Ability to run checked mainly by running docker build --tag=capirca . and docker run capirca, since the current Dockerfile conveniently uses Python 3.6.

To run the tests, a separate Dockerfile.tests was used (but not submitted) with the following changes:

WORKDIR /app

ENTRYPOINT ["python3", "-m", "unittest", "discover", "-s", ".", "-p", "nsxt_test.py", "-v"]

and running the test as follows:

docker build -f Dockerfile.tests --tag capirca:test_nsxt .
docker run capirca:test_nsxt

Fixes #345.