[3.12] gh-116957: configparser: Do post-process values after DuplicateOptionError (GH-116958) (GH-117013)

If you catch DuplicateOptionError / DuplicateSectionError when reading a
config file (the intention is to skip invalid config files) and then
attempt to use the ConfigParser instance, any values it *had* read
successfully so far, were stored as a list instead of string! Later
`get` calls would raise "AttributeError: 'list' object has no attribute
'find'" from somewhere deep in the interpolation code.

(cherry picked from commit b1bc37597f0d36084c4dcb15977fe6d4b9322cd4)