bug#63622: lisp/progmodes/python.el: performance regression introduced by multiline font-lock

[Tom Gillespie]

I have tested the two patches and the performance is dramatically improved.

> One of the problems is that string-delimiter matches even if there is
> a single character preceding the string.

I agree with the assessment about docstring syntax. The fix in Fix-python-
is ok and fixes the font-locking performance in general. The general issue
with the definition of string-delimiter is something that might need to be
considered in the future if another perf issue crops up, but for now I think it
is safe to leave it alone since docstring detection was the only place where
it seems to be causing issues.

While we're here I think [fF]? should probably be included in the prefix since
format strings are also valid docstrings.

Thank you very much for digging into this!
Tom