Pandas: drop a level from a multi-level column index?

If I’ve got a multi-level column index:

>>> cols = pd.MultiIndex.from_tuples([("a", "b"), ("a", "c")])
>>> pd.DataFrame([[1,2], [3,4]], columns=cols)

    a
   ---+--
    b | c
--+---+--
0 | 1 | 2
1 | 3 | 4

How can I drop the “a” level of that index, so I end up with:

    b | c
--+---+--
0 | 1 | 2
1 | 3 | 4