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The tuples should get deallocated when Py_Finalize is called.

It would be good if there was (conditional) statistical
analysis, showing how often no tuple was found because the
number of arguments was too large, and how often no tuple
was found because the candidate was in use.

I think it should be more stack-like, starting off with no
tuples allocated, then returning them inside the needs_free
blocks only if the refcount is 1 (or 2?). This would avoid
degeneralized cases where some function holds onto its
argument tuple indefinitely, thus consuming all 64 tuples.

For the other part, I think it would make the code more
readable if it inlined PyCFunction_Call even more: the test
for NOARGS|O could be integrated into the switch statement
(one case for each), VARARGS and VARARGS|KEYWORDS would both
load the arguments, then call the function directly
(possibly with NULL keywords). OLDARGS should goto either
METH_NOARGS, METH_O, or METH_VARARGS depending on na (if you
don't like goto, modifying flags would work as well).