gh-111784: Fix two segfaults in the elementtree module (GH-113405)

First fix resolve situation when pyexpat module (which contains expat_CAPI
capsule) deallocates before _elementtree, so we need to hold a strong
reference to pyexpat module to.

Second fix resolve situation when module state is deallocated before
deallocation of XMLParser instances, which uses module state to clear
some stuff.