I will be glad to rebase my PR and remove the try/except from the test if you remove the dependency of separator searching code on PyUnicode_AsUTF8AndSize() as you suggest. Or we can go the other way and merge mine first -- whatever you prefer.

Note that technically a difference between C and Python implementation of fromisoformat() will still remain: if a part of the input string before or after the separator contains surrogates, the C code will throw a UnicodeEncodeError while the Python code -- ValueError. But since the former error is a subclass of the latter, I guess it's OK, what do you think?

Also, note that the other discovered C/Python impl difference (for strftime, handled by another try/catch in tests) won't go away, of course, unless someone is ready to fix that as well.