CI: add --diff to black

@ReinhardPrix : This should make a failed job report back what are the actual formatting issues. This would be useful as it seems that black is getting stricter in higher python versions, e.g. the changes here passed black 19.10b0 for me on CIT (python 3.6.8) but fail what the CI is running.

Do you have any concerns that the larger job logs might be a problem?

added 1 commit

• 8ef80f6a - CI: add --diff to black for verbose style critique output

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Oops, it should be black --check --diff, both together, because diff alone gives positive return status even if it finds something to complain about. Commit amended.

changed title from CI: {-change black --check to `black --diff"-} to CI: add --diff to black

changed the description

merged

mentioned in commit a0c86be8