Make a single char from the string uppercase.

def _make_one_char_uppercase(string: str) -> str:
        """Make a single char from the string uppercase."""
        if not isinstance(string, str):
            raise TypeError('string must be a string')

        if Aux.lowercase_count(string) > 0:
            while True:
                cindex = randbelow(len(string))
                if string[cindex].islower():
                    aux = list(string)
                    aux[cindex] = aux[cindex].upper()
                    string = ''.join(aux)
                    break

        return string