[3.6] bpo-30726: PCbuild _elementtree: remove duplicate defines (#2348) (#2349)

[3.6] bpo-30726: PCbuild _elementtree: remove duplicate defines (#2348) (#2349)

* bpo-30726: PCbuild _elementtree: remove duplicate defines (#2348)

bpo-30726, bpo-29591: libexpat 2.2.1 of Modules/expat/ now uses
a winconfig.h configuration file which already defines:

* XML_NS
* XML_DTD
* BYTEORDER=1234
* XML_CONTEXT_BYTES=1024
* HAVE_MEMMOVE

Remove these defines from PCbuild/_elementtree.vcxproj to prevent
compiler warnings.

Co-Authored-By: Jeremy Kloth <jeremy.kloth@gmail.com>
(cherry picked from commit c8fb58bd7917151e63398587a7fc2126db7c26de)

* bpo-30726: Fix elementtree warnings on Windows due to expat upgrade (#2319)

* bpo-30726: Fix elementtree warnings on Windows

Caused by usage of `getenv` which should be safe. And a few integer
truncations which should also be ok.

* bpo-30726: Don't ignore libexpat warnings which haypo intends to fix upstream

(cherry picked from commit 87c65550730a8f85ce339ba197bce4fb7e836619)
(cherry picked from commit d32a05953130fb5cc2d3c0c9fcb20ad0859353f3)