ReDoS: fix unpaired surrogate test

[smowton]

This actually does result in an FP, but this was previously hidden by non-interpretation of '\u' escapes within a raw string.

[smowton]

I note by the way JS has an exclusion in its regex library to recognise when substitution has happened (? or now U+FFFD in the string rep, unicode escape sequence in the source rep), which could be productive to include in Python to prevent these FPs: see @erik-krogh's fix at #6653

[yoff]

Thanks!