[3.4] bpo-29591, bpo-30694: Upgrade Modules/expat to libexpat 2.2.1 (#2164) (#2203)

[3.4] bpo-29591, bpo-30694: Upgrade Modules/expat to libexpat 2.2.1 (#2164) (#2203)

* bpo-29591: Upgrade Modules/expat to libexpat 2.2 (#2164)

* bpo-29591: Upgrade Modules/expat to libexpat 2.2

* bpo-29591: Restore Python changes on expat

* bpo-29591: Remove expat config of unsupported platforms

Remove the configuration (Modules/expat/*config.h) of unsupported
platforms:

* Amiga
* MacOS Classic on PPC32
* Open Watcom

* bpo-29591: Remove useless XML_HAS_SET_HASH_SALT

The XML_HAS_SET_HASH_SALT define of Modules/expat/expat.h became
useless since our local expat copy was upgrade to expat 2.1 (it's now
expat 2.2.0).

(cherry picked from commit 23ec4b57e1359f9c539b8defc317542173ae087e)

* bpo-30694: Upgrade Modules/expat/ to libexpat 2.2.1 (#2300)

New file: Modules/expat/siphash.h.
(cherry picked from commit 5ff7132313eb651107b179d20218dfe5d4e47f13)

* 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)