IPython magic only import things in `"__all__"`

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

Cython/Build/IpythonMagic.py

@@ -93,7 +93,15 @@ class CythonMagics(Magics):
         self._pyximport_installed = False
 
     def _import_all(self, module):
-        for k,v in module.__dict__.items():
+        mdict = module.__dict__
+        if '__all__' in mdict:
+            for k in mdict['__all__']:
+                if k in mdict:
+                    self.shell.push({k:mdict[k]})
+                else:
+                    raise AttributeError("'module' object has no attribute '%s'" % k)
+        else:
+            for k, v in mdict.items():
                 if not k.startswith('__'):
                     self.shell.push({k:v})