(From PSRT list, Sebastian:)

 Please note that the vulnerability fix also added two new functions to
the API that would be great to have xml.parsers.expat expose to the
users for full control.  These are:

- XML_SetBillionLaughsAttackProtectionMaximumAmplification and
- XML_SetBillionLaughsAttackProtectionActivationThreshold

Module xml.parsers.expat.errors and its docs also needs 6 new error code
entries to be complete:

  /* Added in 2.0. */
  38 XML_ERROR_RESERVED_PREFIX_XML
  39 XML_ERROR_RESERVED_PREFIX_XMLNS
  40 XML_ERROR_RESERVED_NAMESPACE_URI

  /* Added in 2.2.1. */
  41 XML_ERROR_INVALID_ARGUMENT

  /* Added in 2.3.0. */
  42 XML_ERROR_NO_BUFFER

  /* Added in 2.4.0. */
  43 XML_ERROR_AMPLIFICATION_LIMIT_BREACH

With regard to the table of vulnerabilities mentioned in the ticket,
please note that vulnerability "quadratic blowup" is also fixed by
>=2.4.0.  Personally, I consider it a flavor of Billion Laughs and all
know variations are covered, including that one.