Currently there is no python api function to assign/remove vertices to specific
vertex group. Traditional `object.vertex_groups[0].add/rna_VertexGroup_vertex_add`
didn't work, because Grease Pencil stores deform verts and vertex group
on individual drawings. Now intorduced a new function
`vertex_group_assign` and `vertex_group_remove` on `Drawing`.
Resolves#135281.
Resolves#134678.
Example (just add GP Suzanne):
```
import bpy
C = bpy.context
drawing = C.active_object.data.layers['Lines'].current_frame().drawing
drawing.vertex_group_assign(vgroup_name="Group", indices_ptr=[2, 6, 10, 18, 19, 20], weight = 0.9)
drawing.vertex_group_remove(vgroup_name="Group", indices_ptr=[2, 6, 10])
```
Pull Request: https://projects.blender.org/blender/blender/pulls/135283
ba2e567efb