BUG : fix  _reshape_2D bug with [(n, 1), ..] input

`cbook._reshape_2D` the input can be _at most_ 2D, however if it
is passed a list of (n, 1) shaped ndarray this condition will
be violated and it fails to work (which is less than great, but
fine from a documentation stand point).  However, boxplot _used_
to work with a list of such ndarrays (and users might expect to
call a (n, 1) shaped array a 1D array).  The added `ravel` makes
sure that even if we get a list of any dimensional ndarrays in, the
output will be a list of (n, ) shaped arrays.

Closes #3220