Black for developers

[RMeli]

This PR explains how developers are supposed to use black to format the code base, if this tool will be adopted by MDAnalysis. In such case this will become a required step of the PR process.

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📚 Documentation preview 📚: https://mdanalysisuserguide--383.org.readthedocs.build/en/383/

[RMeli]

@MDAnalysis/coredevs @marinegor

The companion PR https://github.com/MDAnalysis/mdanalysis/pull/4717 add a black configuration to package and testsuite so that the commands in the documentation

black package
black testsuite

do nothing.

[RMeli]

Thanks @IAlibay. I'll merge this if/when https://github.com/MDAnalysis/mdanalysis/pull/4725 is merged.