[3.13] gh-140593: Fix a memory leak in function `my_ElementDeclHandler` of `pyexpat` (GH-140602) (#140630)

[3.13] gh-140593: Fix a memory leak in function `my_ElementDeclHandler` of `pyexpat` (GH-140602)

Ensure that the memory allocated for the content model
passed to `my_ElementDeclHandler` is freed in all error
paths.

(cherry picked from commit e34a5e33049ce845de646cf24a498766a2da3586)