ConfigParser chokes on conflict markers in stdin and throws AttributeError

Bug report

Bug description:

This is probably a duplicate of bug 107625, but I do not have a recent enough version of Python to check the behaviour.

Whether it is worth specially detecting conflict markers is something the code owners can decide.

To reproduce:

/tmp/py:

#!/usr/bin/python3

import configparser
import sys

sl = configparser.ConfigParser(allow_no_value=True)
sl.read_file(sys.stdin)

/tmp/z:

[list]
<<<<<<<< HEAD   (f12435 blah)
 foo
|||||||| BASE
 bar
========
 baz
>>>>>>>> CHANGE (faa768 blah)

$ </tmp/z /tmp/py
Traceback (most recent call last):
  File "/tmp/py", line 7, in <module>
    sl.read_file(sys.stdin)
  File "/usr/lib/python3.11/configparser.py", line 734, in read_file
    self._read(f, source)
  File "/usr/lib/python3.11/configparser.py", line 1077, in _read
    cursect[optname].append(value)
    ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'append'

Notes:

• sys.stdin seems to be needed to trigger this - deserves further investigation because it may point to a bad assumption or similar in the parser
• allow_no_value is needed to trigger this

Finger of blame:

            if (cursect is not None and optname and
                cur_indent_level > indent_level):
                cursect[optname].append(value)

This does not check that cursect[optname] is present.

CPython versions tested on:

3.10, 3.11, 3.12

Operating systems tested on:

Linux

Commencez dans configparser.py, au niveau de ConfigParser._read, en particulier dans la gestion des continuations autour de la ligne signalée cursect[optname].append(value). Reproduisez le cas avec allow_no_value=True et une entrée contenant des marqueurs de conflit via stdin, puis ajoutez une couverture de régression montrant que l’analyseur ne lève plus d’AttributeError et vérifiez que le comportement existant reste approprié.