Fix `cols` conversion

asccol.py

@@ -9,7 +9,8 @@ class DataSpec:
     """Details about how to parse an aligned data file."""
 
     def __init__(self, cols, leading=0):
-        self.cols = tuple(map(Column._make, cols))
+        cols = tuple(map(Column._make, cols))
+        self.cols = cols
         self.leading = leading
         self.namedtuple = namedtuple('row', (col.name for col in cols))