dist: xenial

language: python

matrix:
  include:
    - python: 3.7
    - python: 3.8
      PYTEST_MYPY=--mypy
    - python: 3.9-dev

install:
  - pip install .[dev]

script:
  - py.test -vvv $PYTEST_MYPY

Add Travis CI configuration, that will enforce tests will always run.
Add Travis CI configuration, that will enforce tests will always run.

This includes testing mypy (but only on Python 3.8, which is currently the
latest stable, since it covers all Python versions).
