I've just started using python-docx and I'm trying to center the contents of my table. I have:

```
table = document.add_table(rows=1, cols=1)
tableHeader = table.rows[0].cells
tableHeader[0].text = 'test'
row_cells = table.add_row().cells
row_cells[0].text = 'example'
table.style = 'MediumGrid3'

```

which outputs a table with the header `test` and the text `example`.

I thought `table.alignment = 1` would work, but it does nothing.

So how do I align all the text to the center?