Fix deprecation warnings about Traitlets tag (#50)

Author: martinRenou

traittypes/traittypes.py

@@ -187,7 +187,8 @@ def __init__(self, default_value=Empty, allow_none=False, dtype=None, **kwargs):
             import pandas as pd
             kwargs['klass'] = pd.DataFrame
         super(DataFrame, self).__init__(
-            default_value=default_value, allow_none=allow_none, dtype=dtype, **kwargs)
+            default_value=default_value, allow_none=allow_none, **kwargs)
+        self.tag(dtype=dtype)
 
 
 class Series(PandasType):
@@ -202,7 +203,8 @@ def __init__(self, default_value=Empty, allow_none=False, dtype=None, **kwargs):
             import pandas as pd
             kwargs['klass'] = pd.Series
         super(Series, self).__init__(
-            default_value=default_value, allow_none=allow_none, dtype=dtype, **kwargs)
+            default_value=default_value, allow_none=allow_none, **kwargs)
+        self.tag(dtype=dtype)
         self.dtype = dtype
 
 
@@ -266,7 +268,8 @@ def __init__(self, default_value=Empty, allow_none=False, dtype=None, **kwargs):
             import xarray as xr
             kwargs['klass'] = xr.Dataset
         super(Dataset, self).__init__(
-            default_value=default_value, allow_none=allow_none, dtype=dtype, **kwargs)
+            default_value=default_value, allow_none=allow_none, **kwargs)
+        self.tag(dtype=dtype)
 
 
 class DataArray(XarrayType):
@@ -281,5 +284,6 @@ def __init__(self, default_value=Empty, allow_none=False, dtype=None, **kwargs):
             import xarray as xr
             kwargs['klass'] = xr.DataArray
         super(DataArray, self).__init__(
-            default_value=default_value, allow_none=allow_none, dtype=dtype, **kwargs)
+            default_value=default_value, allow_none=allow_none, **kwargs)
+        self.tag(dtype=dtype)
         self.dtype = dtype