Update pythreejs.py to silence traitlets warning

Traitlets warning encountered:
...\site-packages\pythreejs\pythreejs.py:56
  C:\Users\genevieb\AppData\Local\conda\conda\envs\ipyvolume-dev\lib\site-packages\pythreejs\pythreejs.py:56: DeprecationWarning: Traits should be given as instances, not types (for example, `Int()`, not `Int`). Passing types is deprecated in traitlets 4.1.
    z = List(CFloat, [0] * 100)

-- Docs: https://docs.pytest.org/en/latest/warnings.html

Author: GenevieveBuckley

pythreejs/pythreejs.py

@@ -53,7 +53,7 @@ class SurfaceGeometry(BufferGeometry):
     """
     A regular grid with heights
     """
-    z = List(CFloat, [0] * 100)
+    z = List(CFloat(), [0] * 100)
     width = CInt(10)
     height = CInt(10)
     width_segments = CInt(10, read_only=True)