Add testcase for sequence unpacking.

tests/basics/seq-unpack.py

+# Basics
+a, b = 1, 2
+print(a, b)
+a, b = (1, 2)
+print(a, b)
+(a, b) = 1, 2
+print(a, b)
+(a, b) = (1, 2)
+print(a, b)
+
+# Tuples/lists are optimized
+a, b = [1, 2]
+print(a, b)
+[a, b] = 100, 200
+print(a, b)
+
+try:
+    a, b, c = (1, 2)
+except ValueError:
+    print("ValueError")
+try:
+    a, b, c = [1, 2, 3, 4]
+except ValueError:
+    print("ValueError")
+
+# Generic iterable object
+a, b, c = range(3)
+print(a, b, c)
+try:
+    a, b, c = range(2)
+except ValueError:
+    print("ValueError")
+try:
+    a, b, c = range(4)
+except ValueError:
+    print("ValueError")