Trivial: Contributing guide should request source compatibility for tests as well.

[aknrdureegaesr]

Environment

Python Version:
3.9.2

Nikola Version:
Git commit e333da4

Operating System:
Linux

Description:

The Contributing Guidelines recommends integrating for code style via flake8 nikola. At the same time, it recommends providing tests. I have found experimentally, via flake8 nikola tests, that those sources are following that code style as well. In my opinion, the guidelines should recommend that for code style integration.