Remove hard pandas dependency for melt by using methodcaller (dask#4719)

* remove hard pandas dependency for melt by using methodcaller

* move import to the top of the file

* whitespace line

dask/dataframe/reshape.py

@@ -8,6 +8,7 @@
 from .utils import (
     is_categorical_dtype, is_scalar, has_known_categories, PANDAS_VERSION
 )
+from ..utils import M
 
 
 ###############################################################
@@ -240,7 +241,7 @@ def melt(frame, id_vars=None, value_vars=None, var_name=None,
 
     from dask.dataframe.core import no_default
 
-    return frame.map_partitions(pd.melt, meta=no_default, id_vars=id_vars,
+    return frame.map_partitions(M.melt, meta=no_default, id_vars=id_vars,
                                 value_vars=value_vars,
                                 var_name=var_name, value_name=value_name,
                                 col_level=col_level, token='melt')