gh-152216: Silence expat compiler warnings

[cdce8p]

Silence expat compiler warnings on macOS with clang.

Infileincludedfrom ./Modules/expat/xmltok.c:309:
./Modules/expat/xmltok_impl.c:283:5: warning: fallthroughannotationinunreachablecode [-Wunreachable-code-fallthrough]
283 | CHECK_NMSTRT_CASES(enc, ptr, end, nextTokPtr)
| ^
./Modules/expat/xmltok_impl.c:116:5: note: expandedfrommacro'CHECK_NMSTRT_CASES'116 | EXPAT_FALLTHROUGH; \
| ^
./Modules/expat/fallthrough.h:45:33: note: expandedfrommacro'EXPAT_FALLTHROUGH'45 | # defineEXPAT_FALLTHROUGH __attribute__((fallthrough))
| ^
...

• Issue: Upgrade bundled Expat to 2.8.2 (e.g. for the fix to CVE-2026-56132 and 12 others) #152216

[StanFromIreland]

As I noted in the issue, we won't be doing this.