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Avoid scanning a substring to compute the maximum character is a good thing, so +1 for the idea. Instead of modifying an existing function, it might be safer to rename it. Even if it is private, a third party module *can* use it outside Python. It is also surprising that you have to specifiy a "maxchar" argument whereas the purpose of the function is to compute the maximum character.
Ex: rename _PyUnicode_FindMaxChar() to _PyUnicode_FindMaxChar2(), and add _PyUnicode_FindMaxChar() as the macro:
#define _PyUnicode_FindMaxChar(str, start, length) _PyUnicode_FindMaxChar2(str, start, length, 127)