Hi Josh. Thanks for the tip on types.MethodType. I've updated the code to use that and the behavior seems to be the same, MethodType does seem a more appropriate way to create a bound method.

Regarding the referenced tickets, I suspect they're not pertinent, as the behavior did work in Python 3.7.1 (confirmed) and 3.7.2 (highly likely), so the regression appears to be in the changes for 3.7.3 (https://docs.python.org/3.7/whatsnew/changelog.html#python-3-7-3-final).