Inside of a module, it is safe to convert a function declared
    cdef int func() except *:
        ...

into

    cdef int func() except? -1

because a) we need an exception return value anyway and b) there will always be an explicit exception check afterwards. Thus, changing the function implementation itself is safe. We must exclude functions that are only declared but not implemented since we do not control their signature and it is not safe to assume a specific exception return value if it is not declared.

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.