Support for Pillow >= 8.3.0

[gyes00205]

As stated in issue 229, I contribute the implementation to check Pillow version. If the version of Pillow >= 8.3.0, the _PALETTE list must consist of all channels for one color followed by the next color (e.g. RGBRGBRGB). Otherwise, all R values must be contiguous in the list before G and B values.