Fix python-shell font-lock cleanup for unclosed quotes (Bug#32390)

Fix python-shell font-lock cleanup for unclosed quotes (Bug#32390)

The problem originating this report was:

-------------
In [15]: "
  File "<ipython-input-15-3b7a06bb1102>", line 1
    "
     ^
SyntaxError: EOL while scanning string literal

In [16]:   string face still here"
-------------

This happens because
python-shell-font-lock-comint-output-filter-function is called twice,
first for the error output and then for the "In [16]: " part.  The
first time python-shell-comint-end-of-output-p returns nil since
we're *not* at the end of an input prompt.  The second time it returns
0 since we're at the end of *just* an input prompt.  So we don't call
python-shell-font-lock-cleanup-buffer either time.

The current code is relying in a very weak rule: it considers "just an
input prompt" to be a continuation prompt.  Another unreliable aspect
of the current rule is that sometimes

    (python-shell-comint-end-of-output-p (ansi-color-filter-apply output))

returns 1 and not 0 for continuation prompts.  In short, the rule does
a very poor job identifying continuations.

* lisp/progmodes/python.el (python-shell-font-lock-cleanup-buffer):
Don't check for empty OUTPUT since python-shell-comint-end-of-output-p
returns nil for that anyway.  Don't check for
python-shell-comint-end-of-output-p returning a specific number
because it's unreliable, just check for any non-nil.  Identify
continuation prompts by looking for "...".