Use platform dependent test for "unpacked_struct_with_arrays" (GH-3621)

tests/buffers/buffmt.pyx

 from __future__ import unicode_literals
+import struct
 
 # Tests buffer format string parsing.
 
@@ -442,7 +443,10 @@ def packed_struct_with_arrays(fmt):
 @testcase
 def unpacked_struct_with_arrays(fmt):
     """
-    >>> unpacked_struct_with_arrays("T{i:a:(8)f:b:f:c:Q:d:(5)i:e:i:f:i:g:xxxx(4)d:h:i:i:}")
+    >>> if struct.calcsize('P') == 8:  # 64 bit
+    ...     unpacked_struct_with_arrays("T{i:a:(8)f:b:f:c:Q:d:(5)i:e:i:f:i:g:xxxx(4)d:h:i:i:}")
+    ... elif struct.calcsize('P') == 4:  # 32 bit
+    ...     unpacked_struct_with_arrays("T{i:a:(8)f:b:f:c:Q:d:(5)i:e:i:f:i:g:(4)d:h:i:i:}")
     """
 
     cdef object[UnpackedStructWithArrays] buf = MockBuffer(