Generates a random string of upper and lowercase letters.
        :type min_chars: int
        :type max_chars: int
        :return: String. Random of random length between min and max characters.

def pystr(self, min_chars=None, max_chars=20):
        """
        Generates a random string of upper and lowercase letters.
        :type min_chars: int
        :type max_chars: int
        :return: String. Random of random length between min and max characters.
        """
        if min_chars is None:
            return "".join(self.random_letters(length=max_chars))
        else:
            assert (
                max_chars >= min_chars), "Maximum length must be greater than or equal to minium length"
            return "".join(
                self.random_letters(
                    length=self.generator.random.randint(min_chars, max_chars),
                ),
            )