Assuming we have an .stl file under filenameSTL we can load the contents of that file using the vtkSTLReader class as such:
readerSTL = vtk.vtkSTLReader()
readerSTL.SetFileName(filenameSTL)
# 'update' the reader i.e. read the .stl file
readerSTL.Update()
polydata = readerSTL.GetOutput()