The current implementation in python-mode fontifies only basic
assignment statements.  It makes mistakes on or entirely misses more
complex ones such as

  [a, b, c] = 1, 2, 3
  a, *b, c = range(10)
  inst.a, inst.b, inst.c = 'foo', 'bar', 'baz'
  (a, b, *c, d) = x, *y = 5, 6, 7, 8, 9

The patch attached below extends the fontification to work correctly
with all these cases.  I tested things as well as I could, but would
appreciate further testing.