Use list slicing instead of extended iterable unpacking

Extended iterable unpacking wasn't added until PEP 3132 and so isn't in
Python 2, which is causing the Python 2.7 tests to fail. While this is
annoying, it is probably worth still supporting Python 2.7 until the
official Python 2 end of life date.

c.f. https://www.python.org/dev/peps/pep-3132/

pyhf/tensor/mxnet_backend.py

@@ -39,9 +39,9 @@ def outer(self, tensor_in_1, tensor_in_2):
         tensor_1_shape = tensor_in_1.shape
         tensor_2_shape = tensor_in_2.shape
         if len(tensor_1_shape) == 1:
-            tensor_1_shape = (*tensor_1_shape, 1)
+            tensor_1_shape = (tensor_1_shape[0], 1)
         if len(tensor_2_shape) == 1:
-            tensor_2_shape = (*tensor_2_shape, 1)
+            tensor_2_shape = (tensor_2_shape[0], 1)
 
         rows1, cols1 = tensor_1_shape
         rows2, cols2 = tensor_2_shape