* bpo-30947, bpo-31170: Update expat from 2.2.1 to 2.2.4

* Upgrade libexpat embedded copy from version 2.2.1 to 2.2.3 to get security
  fixes.

* Update libexpat from 2.2.3 to 2.2.4. Fix copying of partial
  characters for UTF-8 input (libexpat bug 115):
  https://github.com/libexpat/libexpat/issues/115

* Define XML_POOR_ENTROPY when compiling expat