[3.12] gh-115398: Expose Expat >=2.6.0 reparse deferral API (CVE-2023…

…-52425) (GH-115623) (GH-116248)

Allow controlling Expat >=2.6.0 reparse deferral (CVE-2023-52425) by adding five new methods:

- `xml.etree.ElementTree.XMLParser.flush`
- `xml.etree.ElementTree.XMLPullParser.flush`
- `xml.parsers.expat.xmlparser.GetReparseDeferralEnabled`
- `xml.parsers.expat.xmlparser.SetReparseDeferralEnabled`
- `xml.sax.expatreader.ExpatParser.flush`

Based on the "flush" idea from python/cpython#115138 (comment) .

- Please treat as a security fix related to CVE-2023-52425.

(cherry picked from commit 6a95676bb526261434dd068d6c49927c44d24a9b)
(cherry picked from commit 73807eb634315f70a464a18feaae33d9e065de09)
(cherry picked from commit eda2963378a3c292cf6bb202bb00e94e46ee6d90)

---------

Includes code suggested-by: Snild Dolkow <[email protected]>
and by core dev Serhiy Storchaka.
Co-authored-by: Gregory P. Smith <[email protected]>

CPython-sync-commit-latest: 0a01ed6c2a116bd3e174fce33c21d84d650de569