[BUG] Error when writing SU2 boundary data

[vtpasquale]

Describe the bug
Trying to roundtrip the stock .su2 file fails

To Reproduce
This is the example mesh file: https://su2code.github.io/docs_files/square.su2

Running this code

import meshio
m = meshio.read("square.su2")
m.write("square_rt.su2")`

Results in this error

    File ~/.local/lib/python3.10/site-packages/meshio/su2/_su2.py:340 in write
    for index, (cell_type, data) in enumerate(mesh.cells):

    TypeError: cannot unpack non-iterable CellBlock object

Diagnose
I may ask you to cut and paste the output of the following command.

pip freeze | grep meshio

meshio==5.3.4

To Fix
I replaced line 340 with these three lines:

            for index, cell in enumerate(mesh.cells):
                cell_type = cell.type
                data = cell.data

I'll submit a pull request from my fork.