Sets a AttrList; the attributes in the list are applied to the label text.
The attributes set with this function will be applied and merged with any other attributes previously effected by way of the use_underline or use_markup properties. While it is not recommended to mix markup strings with manually set attributes, if you must; know that the attributes will be applied to the label after the markup string is parsed.
| this |
a Label |
| attrs |
a AttrList, or null |