IPython/utils/tests/test_path.py: Only use `encode` for system functions that cannot handle unicode.

IPython/utils/tests/test_path.py

@@ -430,12 +430,12 b' def test_unescape_glob():'
 
 def test_ensure_dir_exists():
     with TemporaryDirectory() as td:
-        d = os.path.join(td, u'∂ir').encode("utf8")
+        d = os.path.join(td, u'∂ir')
         path.ensure_dir_exists(d) # create it
-        assert os.path.isdir(d)
+        assert os.path.isdir(d.encode("utf8"))
         path.ensure_dir_exists(d) # no-op
-        f = os.path.join(td, u'ƒile').encode("utf8")
-        open(f, 'w').close() # touch
+        f = os.path.join(td, u'ƒile')
+        open(f.encode("utf8"), 'w').close() # touch
         with nt.assert_raises(IOError):
             path.ensure_dir_exists(f)