configparser: reading an invalid file triggers ParsingError *and* mutations

Bug report

A clear and concise description of what the bug is.
Include a minimal, reproducible example (https://stackoverflow.com/help/minimal-reproducible-example), if possible.

Your environment

• CPython versions tested on: 3.11.0b5
• Operating system and architecture: x86_64

here is a simple example:

>>> import configparser
>>> cfg = configparser.ConfigParser()
>>> cfg.read_string('[foo]\nx = 1\n...\n')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.11/configparser.py", line 738, in read_string
    self.read_file(sfile, source)
  File "/usr/lib/python3.11/configparser.py", line 733, in read_file
    self._read(f, source)
  File "/usr/lib/python3.11/configparser.py", line 1131, in _read
    raise e
configparser.ParsingError: Source contains parsing errors: '<string>'
	[line  3]: '...\n'
>>> cfg['foo']['x']
'1'

I expect reading to be an all-or-nothing operation -- if the file contains a syntax error it shouldn't modify the ConfigParser object with half of the file's contents

Start with the configparser.py implementation shown in the traceback and reproduce the read_string example from the issue. Trace how _read handles valid lines before raising ParsingError, then add or locate focused tests for the failure case. Done means an invalid input raises ParsingError without leaving partial contents in the ConfigParser object.