I didn't mean single place. I meant changing all % usages.

@methane: I didn't mean single place. I meant changing all % usages.

Well, the suggested change feature commits exactly what you suggested (I don't actually know if I can change it at all), and I didn't look any further, trusting your review.
Could you please do the task to completion? There may be yet other small things to fix or something.

and I didn't look any further, trusting your review.

Please don't trust me as such. I hope contributors is not robot just applying suggested change. I hope contributors understand and grow up by review.

Could you please do the task to completion? There may be yet other small things to fix or something.

At a code level, looks good me.
But I'm not sure that adding more PYTHON* env vars is really good thing for this particular case...

Adding more and more envvar makes it harder to users read manual and find option they needed by their self.