[line-directive] Escape literal '['
New versions of Python warn if the literal is not escaped. Specifically:
"Support for nested sets and set operations in regular expressions as in
Unicode Technical Standard #18 might be added in the future. This would
change the syntax. To facilitate this future change a FutureWarning will
be raised in ambiguous cases for the time being. That include sets
starting with a literal '[' or containing literal character sequences
'--', '&&', '~~', and '||'. To avoid a warning, escape them with a
backslash. (Contributed by Serhiy Storchaka in bpo-30349.)"
https://docs.python.org/dev/whatsnew/3.7.html
