Improve flake8 command in CONTRIBUTING.rst. Fix #3716. (#3717) #368

Job	Run time
Build demo site with base requirements (3.11)	24s
Linting (flake8, pydocstyle) (3.11)	29s
Baseline testing (Python 3.11 on ubuntu-latest)	44s
Nikola tests (Python 3.7 on ubuntu-latest)	2m 34s
Nikola tests (Python 3.8 on ubuntu-latest)	2m 27s
Nikola tests (Python 3.9 on ubuntu-latest)	3m 22s
Nikola tests (Python 3.10 on ubuntu-latest)	2m 27s
Nikola tests (Python 3.11 on ubuntu-latest)	2m 45s
Nikola tests (Python 3.11 on macos-latest)	3m 17s
Nikola tests (Python 3.11 on windows-latest)	5m 4s
	23m 33s

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