Stop suggesting the single-threaded variant for running tests

[pradyunsg]

Currently, the first suggestion in our Getting Started guide, for running tests is tox -e py36. This is sub-optimal since contributors run this command directly, without looking at the suggestion immediately after to run the tests in parallel. This can end up taking a lot of time, which isn't ideal for time-constrained events like development sprints and isn't a great contribution experience either.

It would be a good idea to move the tox -e py36 -- -n auto suggestion, to be first, rephrasing the sentences to more clearly indicate that it is preferable to run tests in parallel when running multiple tests.

Relevant file: docs/html/development/getting-started.rst
Relevant documentation (built): https://pip.pypa.io/en/latest/development/getting-started/#running-tests

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[KoundinyaSi]

Hey!
I would like to work on this.