[python][feature-request] `GradingPrimary` : fail to assertEqual 2 similar instances

Hello, same as in #1641, seems 2 similar `ocio.GradingPrimary` instance doesn't assert equal.

```python
import unittest

import PyOpenColorIO as ocio

class OcioGradingPrimary(unittest.TestCase):
def test_equal1(self):

gp1 = ocio.GradingPrimary(ocio.GRADING_LIN)
gp1.saturation = 0.3

gp2 = ocio.GradingPrimary(ocio.GRADING_LIN)
gp2.saturation = 0.3

self.assertEqual(gp1, gp2)

def test_equal2(self):

gp1 = ocio.GradingPrimary(ocio.GRADING_LOG)
gp1.contrast = ocio.GradingRGBM(0.5, 0.5, 0.5, 0.0)

gp2 = ocio.GradingPrimary(ocio.GRADING_LOG)
gp1.contrast = ocio.GradingRGBM(0.5, 0.5, 0.5, 0.0)

self.assertEqual(gp1, gp2)

if __name__ == "__main__":
unittest.main()
```

CONTEXT: OCIO 2.1.1

I assume it's not an expected behaviour?
Cheers.
Liam.