apacheGH-40153: [Python] Update size assumptions for 32-bit platforms (…

…apache#40165)

### Rationale for this change

This fixes two tests on 32-bit platforms (tested on x86 specifically).

### What changes are included in this PR?

- update the `pd.object_` size assumption to 4 bytes on 32-bit platforms
- update the `pa.schema` size assumptions to be twice smaller on 32-bit platforms

### Are these changes tested?

The changes fix tests.

### Are there any user-facing changes?

Only test fixes.

* Closes: apache#40153

Authored-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

python/pyarrow/tests/test_pandas.py

@@ -2608,8 +2608,9 @@ def test_from_numpy_nested(self):
                                        ('yy', np.bool_)])),
                        ('y', np.int16),
                        ('z', np.object_)])
-        # Note: itemsize is not a multiple of sizeof(object)
-        assert dt.itemsize == 12
+        # Note: itemsize is not necessarily a multiple of sizeof(object)
+        # object_ is 8 bytes on 64-bit systems, 4 bytes on 32-bit systems
+        assert dt.itemsize == (12 if sys.maxsize > 2**32 else 8)
         ty = pa.struct([pa.field('x', pa.struct([pa.field('xx', pa.int8()),
                                                  pa.field('yy', pa.bool_())])),
                         pa.field('y', pa.int16()),

python/pyarrow/tests/test_schema.py

@@ -681,7 +681,8 @@ def test_schema_sizeof():
         pa.field('bar', pa.string()),
     ])
 
-    assert sys.getsizeof(schema) > 30
+    # Note: pa.schema is twice as large on 64-bit systems
+    assert sys.getsizeof(schema) > (30 if sys.maxsize > 2**32 else 15)
 
     schema2 = schema.with_metadata({"key": "some metadata"})
     assert sys.getsizeof(schema2) > sys.getsizeof(schema)