This code in sre_parse (line 738 and down):

                            warnings.warn(
                                'Flags not at the start of the expression %s%s' % (
                                    source.string[:20],  # truncate long regexes
                                    ' (truncated)' if len(source.string) > 20 else '',
                                ),
                                DeprecationWarning, stacklevel=7
                            )


triggers a BytesWarning if you do `_line_re = re.compile(br'^(.*?)$(?m)')`.

The fix should be simple; I can try to get a PR in over the next few days.