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Looks quite obscure, a hack to avoid the bad effects of the
previous hack of temporarily decreasing the recursion depth
(which no other piece of code does).  I'd be much more
confident that I'm looking at correct code if we used a more
explicit approach.  What about a prebuilt RuntimeError
instance with the message "maximum recursion depth
exceeded", similar to the prebuilt MemoryError instance?

Then at least PyObject_Call() could raise this instance
directly, with PyErr_SetObject().  We'd get an
already-normalized exception in this way, and remove any
need for tstate recursion_depth mangling.