Fix definition of multiple ']' in header

[mark99i]

Fix definition of multiple ']' in header in configparser

in old version in example header is "[i love [python] lang]"
parse as "i love [python", only up to the first character ']'.

once fixed, multiple ']' can be used and this will not cause the header to be clipped.
now saving works correctly, but reading does not

[the-knights-who-say-ni]

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[brandtbucher]

Sorry @mark99i, but I think this PR should be closed in favor of #17129, since it includes tests and doesn't break handling on the empty case.

Please make sure to include the "bpo-#####: " prefix in the PR title next time so that it properly links to the issue and others can see that you've begun work.

[brandtbucher]

Thanks for your time though... definitely looking forward to seeing more contributions from you in the future!