traitsui 6.1.2/6.1.3 TabularEditor column width issue

Setting the TabularEditor/TabularAdapter column width in traitsui 6.1.2/6.1.3 doesn't quite work right. The last column ends up being much wider than it should be.

Test code:
```
from traits.api import HasTraits, List, Unicode
from traitsui.api import View, UItem, TabularEditor
from traitsui.tabular_adapter import TabularAdapter

class Demo(HasTraits):
table = List(List(Unicode), [['A', 'B'], ['C', 'D'], ['E', 'F']])

traits_view = View(
UItem(
'table',
editor=TabularEditor(
adapter=TabularAdapter(
columns=['1', '2'],
width=200,
),
),
),
width=500,
height=500,
)

Demo().configure_traits()
```

In traitsui 6.1.1:

In traitsui 6.1.2/6.1.3: