Moved ipython entry point to cython package

`load_entry_point()` is on Cython/__init__.py,
so::   %load_ext Cython is all you need to load the magic.

Cython/Build/IpythonMagic.py

@@ -339,7 +339,3 @@ __doc__ = __doc__.format(
                 CYTHON_INLINE_DOC = dedent(CythonMagics.cython_inline.__doc__),
                 CYTHON_PYXIMPORT_DOC = dedent(CythonMagics.cython_pyximport.__doc__),
 )
-
-def load_ipython_extension(ip):
-    """Load the extension in IPython."""
-    ip.register_magics(CythonMagics)

Cython/__init__.py

@@ -2,3 +2,9 @@ from Cython.Shadow import __version__
 
 # Void cython.* directives (for case insensitive operating systems).
 from Cython.Shadow import *
+
+
+def load_ipython_extension(ip):
+    """Load the extension in IPython."""
+    from Cython.Build.IpythonMagic import CythonMagics
+    ip.register_magics(CythonMagics)