* 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 23ec4b57e1)