Clean up module import handling in PyInit__core - #1081

Did a deep pass on bd95764 (code walk of every failure path + an empirical check). Substance looks good, I'd like to get this in.

Both claimed leaks are real and fixed. On the base, the uuid and decimal blocks are the only two that never Py_DECREF(temp_module) (the reference is overwritten by the next PyImport_ImportModule), and they're exactly what LSan would flag on a plain import. Verified empirically too: with the merge-base and this branch built from the same parent, sys.getrefcount(sys.modules["uuid"]) and ...["decimal"] drop from 4 to 3 while every other module stays identical. Beyond those two, the unified error label also closes the ~20 conditional error paths that used to drop a live temp_module on early return NULL - walked all ten import blocks, the ownership chain converges on every path, no double-DECREF (the SET_TEMP_OBJ/IMPORT_TEMP_MODULE XDECREF-before-replace handles reentry, and the error label's XDECREFs are NULL-safe for the early PyType_Ready gotos). Full unit suite is green on a local build.

On the open items:

• The conflict with main is a single hunk from #962: keep SET_REF(convert_generic_alias, "convert_generic_alias") in the _utils block and drop the now-redundant Py_DECREF(temp_module). Nothing from #1052/#1097/#1109 touches PyInit__core, so it's a one-line resolution.
• The EnumMeta/ABCMeta PyType_Check removal you mentioned trying (and ofek pre-approved with "if CI passes... feel free to merge") never got pushed. Either way works for me - push the removal here, or merge as-is and drop them in a follow-up. I'd just rather not let a green refactor sit for another two weeks.

After the rebase this is an approve from me.

Nice! Two minor suggestions :)