[3.10] gh-115398: Expose Expat >=2.6.0 reparse deferral API (CVE-2023-52425) (GH-115623) (GH-116270)

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 https://github.com/python/cpython/pull/115138#issuecomment-1932444270 .

Includes code suggested-by: Snild Dolkow <snild@sony.com>
and by core dev Serhiy Storchaka.

Co-authored-by: Gregory P. Smith <greg@krypto.org>