A stripped down script that creates a single poly mesh. That is all.

simple_mesh_script.py

import bpy
import bmesh

if __name__ == '__main__':
    verts = [(-5, 0, 0),
              (5, 0, 0),
              (2, 5, 0),]
    
    mesh = bpy.data.meshes.new("mymesh")
    bm = bmesh.new()
    
    for vert in verts:
        bm.verts.new(vert)
    
    bm.verts.ensure_lookup_table()
    
    faces = [[0,1,2], ]
    bm.faces.new([bm.verts[0], bm.verts[1], bm.verts[2]])
    bm.to_mesh(mesh)
    mesh.update()
    ob = bpy.data.objects.new("meshob", object_data=mesh)
    scn = bpy.context.scene.objects.link(ob)