Avoid using PySequence_ITEM() on METH_O calls as it involves much more overhead than PyTuple_GET_ITEM().
Also assume that we do not need to keep an extra reference to an item that we took from a call args tuple because someone else should at least hold a reference to that tuple.

Clean up Pythran setup in cythonize() and make it a little more robust by also looking at the extension, in case it already has the "np_pythran" option set.

Implement a "--pgo" option for the Jupyter magic that applies profile guided optimisation during C compilation.

Update README on pyximport

Closes #1859.

Closes #1860.

Release buffer on validation errors. My guess is that we previously just leaked the buffer owner in that case.

Closes #1858.

Check result of "declare_builtin()" because it might a) not have been found and b) not actually be a builtin. In both cases, it's actually not a literal.