Consider preventing multiple lines in split_before_newlines

[Poikilos]

Probably not, but would require the test change (in str and bytes versions of the test):

replace:

                self.assertEqual(split_before_newlines(" a\n "), (" a", "\n "))
                self.assertEqual(split_before_newlines("a \n "), ("a ", "\n "))

with

        # Pairs that should be rejected (allowing more lines causes more complexity)
        bad_pairs = [
            (" a\n ", (" a", "\n ")),
            ("a \n ", ("a ", "\n ")),
        ]
        for bad_pair in bad_pairs:
            arg, goodTuple = bad_pair
            good_exception = False
            try:
                self.assertEqual(split_before_newlines(arg), goodTuple)
            except ValueError as ex:
                if "more than one line" in str(ex):
                    good_exception = True
                else:
                    raise
            if not good_exception:
                raise AssertionError(
                    "split_before_newlines should reject more than one line"
                )