Note: Uncommenting the line that forces Py_TPFLAGS_HEAPTYPE isn't enough, since it looks like the PyHeapTypeObject fields aren't initialized properly, causing seg faults if you access, for example, __name__/__qualname__ (or print the type's repr, which implicitly accesses same):

    python -c "import testnewtype; Foo = testnewtype.makeseq('Foo', ['x', 'y']); print(Foo.__name__)"

The type behaves properly otherwise (you can make instances, access values on them), but crashing on repr is probably poor form. :-)