fixup! Variable: Add sparse attribute

Author: nikicc

Orange/data/variable.py

@@ -260,10 +260,10 @@ class Variable(Reprable, metaclass=VariableMeta):
         Non-primitive variables must redefine it to return `None`.
 
     .. attribute:: sparse
-    
+
         A flag about sparsity of the variable. When set, the variable suggests
         it is should be stored in a sparse matrix. 
-    
+
     .. attribute:: source_variable
 
         An optional descriptor of the source variable - if any - from which