PyPy3: make special dict methods work

The .iterkeys(), .itervalues() and .iteritems() methods are accepted in Cython
even when targetting Python 3.x.  However, this is implemented by a shortcut
inside Optimize.c that is explicitly disabled on PyPy.  This fixes the
situation.

Let IPython magic only import things in `"__all__"`

skip single underscore
minor restructure

IPython magic only import things in `"__all__"`

Make the IPython cell magic only import things in `"__all__"` when `"__all__"` exits.

Fix TODO: how to display this not as code but verbatim?

Pass first arg of visitproc as PyObject*

Pass metaclass as PyObject* (and not PyTypeObject*)

Update language_basics.rst: corrected an example

Correct mistake in doc `early_binding_for_speed.rst`

corrected an example

IPython notebook to Jupyter notebook in docs

This allows return types such as std::map<K, V>.

Fixes #1599

This fixes #1562

Add a comment about using setuptools and Cython together when passing Extension objects into cythonize() or distutils.
Resolves #338.

Encode relative path to working tree in code paths when possible.

Fix Python 2 and 3 have diffrent default encoding

Closes #1594.