Config with a python build and a matrix only runs the matrix

If I try to build using this config file only the 3.8-dev job runs although I’d expect 3.7 to be build as well.

cache:
  pip: true
  timeout: 1000

language: python

python: 
  - 3.7

matrix:
  fast_finish: true
  include:
    - python: 3.8-dev
      dist: xenial
      sudo: true
      before_install:
        - pip install git+https://github.com/astropy/astropy@master
  allow_failures:
    - python: 3.8-dev

notifications:
  email: false

# repo branches to test
branches:
  - master

install:
  - pip install -U pip wheel --quiet
  - pip install --upgrade setuptools --quiet
  - pip install pytest
  - pip install pytest-coverage
  - pip install coveralls
  - pip install .[dev]

script:
  - pytest --cov basecam --cov-report html

after_success:
  - coveralls

However, if I change the python key to

python:
  - 3.6
  - 3.7

Then all 3.6, 3.7, and 3.8-dev run.

That’s not very intuitive IMHO but at least it should be documented somewhere. If it is I was not able to find it.

Awesome! Thanks.

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