oxml: use templating for CT_Tbl.new()
acpt: add tbl-col-props.feature

With initial scenario outline for column width
tbl: add _Column.width getter

Also, sequence Length methods in alpha order
acpt: add scenario for _Column.width setter
xmlch: fix discovered bug in OptionalAttribute

Didn’t delete optional attribute on assign None when attribute has
namespace prefix. Lookup of attribute needs to be on clark_name, not
attr_name.
tbl: add _Column.width setter
py3: fix Python 3 compatibility on xmlchemy