plistlib inherits expat's newline-normalizing behavior, which can corrupt data in XML plists

Bug report

Bug description:

This is just a special case of #84561 but it came up in the specific circumstance of XML plists in MOPUp, here: https://github.com/glyph/MOPUp/pull/448/files#r2280646662

I also discovered that it is possible to get the unmodified data, by changing plistlib._PlistParser.handle_data to look like this:

    def handle_data(self, data):
        ctx = self.parser.GetInputContext()
        if data == '\n' and ctx[0:1] != b'\n':
            data = ctx[0:1].decode("utf-8")
        self.data.append(data)

so we are not totally bound by the whims of Expat here; these types of bug could be fixed.

CPython versions tested on:

3.12, 3.13, 3.11

Operating systems tested on:

macOS

Linked PRs

• gh-148305

Start with plistlib._PlistParser.handle_data and the input-context behavior described in the report. Verify how XML plist data is currently normalized and ensure unmodified data is preserved; check linked PR gh-148305 before starting because this work may already be underway.