Fix flake8 E275 missing whitespace after keyword warnings

[nyalldawson]

No description provided.

[rouault]

I see a number of instances where the old version before your fix was return' fail', so the space character was wrongly put in the string. That should likely be corrected, although it probably doesn't matter much now ('fail' was a special recognized string before we pytest'ify autotest), or more likely those return 'fail' should be converted to a pytest assertion