Getting started doc should include Python 3.7

[okken]

Thanks for submitting an issue!

Here's a quick checklist in what to include:

• Include a detailed description of the bug or suggestion
  This link: https://docs.pytest.org/en/latest/getting-started.html
  Includes this text: "Pythons: Python 2.7, 3.4, 3.5, 3.6, Jython, PyPy-2.3"
  Should be extended to include 3.7

• pip list of the virtual environment you are using
  N/A

• pytest and operating system versions
  N/A

• Minimal example if possible
  N/A

[nicoddemus]

Thanks @okken! Would you like to open a PR with that? 😉

Simple changes like this can be done directly in GH's web UI btw.

[okken]

sure. I'll give it a shot.