Simply by inspecting mesh_from_file
In [1]: mesh_from_file
Out[1]:
<meshio mesh object>
Number of points: 64
Number of cells:
line: 10
line: 20
line: 10
line: 9
line: 3
line: 3
line: 9
Cell sets: Interior, Left, Right, Neutral, gmsh:bounding_entities
Point data: gmsh:dim_tags
Cell data: gmsh:physical, gmsh:geometrical
Field data: Interior, Left, Right, Neutral
you see that there are no triangular cells in your mesh.
This is because you have not used any Physical Surface in pygmsh, i.e.
adding:
model.add_physical([interior], "Interior Surface")
resolves the issue.