After updating the patch, I noticed that deletion of self.context_use_ps1 from the editor call to y.find_good_parse_start was no longer part of the diff.  This is because 3.8 changeset 6bdc4dee01788599808c7858e2fe9fdd72cf6792 for #35610 (backported only to 3.7) already did so. The remaining substantive change other than new tests is the removal of the unused default value None in the signature of the function and unused code triggered by 'if not arg'. 

I don't remember how well I tested that never exiting find...start early left indentations unchanged.  This would require comparing behavior to 3.6.  It would make more sense to check that the behavior and new tests conform to PEP8.