Relax over-strict alignment test on small arrays

This alignment test was introduced in gh-140557 with the intention of
preventing empty-allocation optimisations from introducing odd-aligned
pointers (as had previously been observed in such an optimisation for
the empty `bytearray`, used in the Pickle v5 protocol).  The test was
over-specified, however, and compared to the maximum platform alignment,
even though the requirements on allocators in general is only that the
allocation is aligned for all data types that would in the requested
allocation.

This test could fail on 32-bit platforms using `mimalloc` for
single-byte array elements (e.g. 'B'), as the tiny allocation region
only enforced 4-byte alignment, not the 8-byte maximum platform
alignment of the `double` type.

Author: jakelishman

Lib/test/test_buffer.py

@@ -4465,9 +4465,9 @@ def test_bytearray_alignment(self):
     @support.cpython_only
     @unittest.skipIf(_testcapi is None, "requires _testcapi")
     def test_array_alignment(self):
-        # gh-140557: pointer alignment of buffers including empty allocation
-        # should match the maximum array alignment.
-        align = max(struct.calcsize(fmt) for fmt in ARRAY)
+        # gh-140557: pointer alignment of empty buffers should be at least
+        # to `size_t`.
+        align = struct.calcsize("N")
         cases = [array.array(fmt) for fmt in ARRAY]
         # Empty arrays
         self.assertEqual(
@@ -4476,9 +4476,11 @@ def test_array_alignment(self):
         )
         for case in cases:
             case.append(0)
-        # Allocated arrays
+        # Allocated arrays. The minimum alignment is now governed by regular
+        # allocator rules; aligned for any type that fits in the allocation.
         self.assertEqual(
-            [_testcapi.buffer_pointer_as_int(case) % align for case in cases],
+            [_testcapi.buffer_pointer_as_int(case) % case.itemsize
+             for case in cases],
             [0] * len(cases),
         )