Fix %env magic on Python 3.

Closes gh-1201

IPython/core/magic.py

@@ -3006,7 +3006,7 b' Defaulting color scheme to \'NoColor\'"""'
     def magic_env(self, parameter_s=''):
         """List environment variables."""
 
-        return os.environ.data
+        return dict(os.environ)
 
     def magic_pushd(self, parameter_s=''):
         """Place the current dir on stack and change directory.

IPython/core/tests/test_magic.py

@@ -467,3 +467,6 b' def test_notebook_reformat_json():'
         _ip.magic("notebook -f ipynb %s" % infile)
         _ip.magic("notebook -f json %s" % infile)
 
+def test_env():
+    env = _ip.magic("env")
+    assert isinstance(env, dict), type(env)