This warning is present in the Travis builds for master and 3.7.

Python/hamt.c: In function ‘_PyHamt_Without’:
Python/hamt.c:2371:76: warning: ‘new_root’ may be used uninitialized in this function [-Wmaybe-uninitialized]
Py_DECREF(new_root);
^
Python/hamt.c: In function ‘hamt_py_delete’:
Python/hamt.c:2371:76: warning: ‘new_root’ may be used uninitialized in this function [-Wmaybe-uninitialized]
Python/hamt.c:2351:17: note: ‘new_root’ was declared here
PyHamtNode *new_root;