gh-126624: Expose error code ``XML_ERROR_NOT_STARTED`` of Expat >=2.6.4 (#126625)

Expose error code ``XML_ERROR_NOT_STARTED`` in `xml.parsers.expat.errors` which was
introduced in Expat 2.6.4.


Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>