[3.6] bpo-30947: Update libexpat from 2.2.1 to 2.2.3 (#3106)

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• bpo-30947: Update libexpat from 2.2.1 to 2.2.3

• Add NEWS entry

• Add new loadlibrary.c

• expat_external.h: restore include "pyexpatns.h"

• PCbuild: add expat/loadlibrary.c

• Define XML_POOR_ENTROPY to compile expat

(cherry picked from commit 93d0cb5)

https://bugs.python.org/issue30947

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The AppVeyor failure seems unrelated:

======================================================================
FAIL: test_s_option (test.test_site.HelperFunctionsTests)
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Traceback (most recent call last):
  File "C:\projects\cpython\lib\test\test_site.py", line 186, in test_s_option
    self.assertIn(usersite, sys.path)
AssertionError: 'C:\\Users\\appveyor\\AppData\\Roaming\\Python\\Python36\\site-packages' not found in ['', 'C:\\projects\\cpython\\PCbuild\\win32\\python36.zip', 'C:\\projects\\cpython\\DLLs', 'C:\\projects\\cpython\\lib', 'C:\\projects\\cpython\\PCbuild\\win32', 'C:\\projects\\cpython', 'C:\\projects\\cpython\\lib\\site-packages']