build:
    image: latest

python:
    version: 3.6
    pip_install: true
    extra_requirements:
        - read-the-docs

build:
    image: latest

python:
    version: 3.7
    pip_install: true
    extra_requirements:
        - read-the-docs

Update "Python" version on RtD.
Update "Python" version on RtD.