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

@@ -297,7 +297,7 @@ class CythonMagics(Magics):
 
     def _clear_distutils_mkpath_cache(self):
         """clear distutils mkpath cache
-        
+
         prevents distutils from skipping re-creation of dirs that have been removed
         """
         try:
@@ -306,7 +306,7 @@ class CythonMagics(Magics):
             pass
         else:
             _path_created.clear()
-    
+
     def _get_build_extension(self):
         self._clear_distutils_mkpath_cache()
         dist = Distribution()
@@ -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)