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.

Add missing special signature for "__getattribute__" method in order to enforce it and mark the method as special.

Replace closure class special casing from slot generation code and replace it with saner and more general "is_special" check.

Repair slot signature typing for "__richcmp__" special method after adding synthesis from "__eq__" etc. methods.
Add test for "__richcmp__" override of existing "__eq__" etc. methods.

Generate "tp_richcompare" slot when "__eq__" and/or its friends are defined but "__richcmp__" is not.
Closes #690.

Implement @cython.exceptval() decorator to make the "except x" signature declaration available in pure Python mode.
Closes #1653.

Need to delay annotation typing after the "as_cfunction()" conversion in AdjustDefByDirectives transform as we do not have an 'env' yet.