gh-115398: Suggest use of hasattr with checking for 3.13 Expat API availability (GH-116278)

Suggest use of "hasattr" with checking for 3.13 Expat API availability

(cherry picked from commit 73807eb)

Author: hartwork

Doc/library/pyexpat.rst

@@ -217,6 +217,11 @@ XMLParser Objects
    Calling ``SetReparseDeferralEnabled(True)`` allows re-enabling reparse
    deferral.
 
+   Note that :meth:`SetReparseDeferralEnabled` has been backported to some
+   prior releases of CPython as a security fix.  Check for availability of
+   :meth:`SetReparseDeferralEnabled` using :func:`hasattr` if used in code
+   running across a variety of Python versions.
+
    .. versionadded:: 3.12.3
 
 .. method:: xmlparser.GetReparseDeferralEnabled()

Doc/library/xml.etree.elementtree.rst

@@ -1397,6 +1397,11 @@ XMLParser Objects
       Disabling reparse deferral has security consequences; please see
       :meth:`xml.parsers.expat.xmlparser.SetReparseDeferralEnabled` for details.
 
+      Note that :meth:`flush` has been backported to some prior releases of
+      CPython as a security fix.  Check for availability of :meth:`flush`
+      using :func:`hasattr` if used in code running across a variety of Python
+      versions.
+
       .. versionadded:: 3.12.3
 
 
@@ -1470,6 +1475,11 @@ XMLPullParser Objects
       Disabling reparse deferral has security consequences; please see
       :meth:`xml.parsers.expat.xmlparser.SetReparseDeferralEnabled` for details.
 
+      Note that :meth:`flush` has been backported to some prior releases of
+      CPython as a security fix.  Check for availability of :meth:`flush`
+      using :func:`hasattr` if used in code running across a variety of Python
+      versions.
+
       .. versionadded:: 3.12.3
 
    .. method:: close()