apacheGH-45570: [Python] Allow Decimal32/64Array.to_pandas (apache#45571)

### Rationale for this change

Enables converting `Decimal32Array` and `Decimal64Array` to pandas

### What changes are included in this PR?

Adds `Type::DECIMAL32` and `Type::DECIMAL64` as convertible types to pandas

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes

closes apache#45570

* GitHub Issue: apache#45570

Lead-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>
Co-authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: mroeschke

python/pyarrow/src/arrow/python/arrow_to_pandas.cc

@@ -2123,6 +2123,8 @@ static Status GetPandasWriterType(const ChunkedArray& data, const PandasOptions&
     case Type::STRUCT:                   // fall through
     case Type::TIME32:                   // fall through
     case Type::TIME64:                   // fall through
+    case Type::DECIMAL32:                // fall through
+    case Type::DECIMAL64:                // fall through
     case Type::DECIMAL128:               // fall through
     case Type::DECIMAL256:               // fall through
     case Type::INTERVAL_MONTH_DAY_NANO:  // fall through

python/pyarrow/tests/test_pandas.py

@@ -2041,6 +2041,19 @@ def test_strided_objects(self, tmpdir):
         df = pd.DataFrame.from_dict(data)
         _check_pandas_roundtrip(df)
 
+    @pytest.mark.parametrize("typ", [
+        pa.decimal32,
+        pa.decimal64,
+        pa.decimal128,
+        pa.decimal256,
+    ])
+    def test_decimal_array_to_pandas(self, typ):
+        data = [decimal.Decimal('3.14'), None]
+        arr = pa.array(data, type=typ(3, 2))
+        result = arr.to_pandas()
+        expected = pd.Series(data)
+        tm.assert_series_equal(result, expected)
+
 
 class TestConvertListTypes:
     """