[3.11] gh-115398: Expose Expat >=2.6.0 reparse deferral API (CVE-2023-52425) (GH-115623) (#116268)

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 #115138 (comment) .

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

(cherry picked from commit 6a95676)
(cherry picked from commit 73807eb)
(cherry picked from commit eda2963)

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Includes code suggested-by: Snild Dolkow <snild@sony.com>
and by core dev Serhiy Storchaka.
Co-authored-by: Gregory P. Smith <greg@krypto.org>