Deprecate BGR;15, BGR;16 and BGR;24 modes

[radarhere]

Helps #2228

[Yay295]

tobytes(), frombytes(), and frombuffer() can also take mode and rawmode parameters through their *args parameter.

[radarhere]

• frombuffer() calls frombytes(), unless the mode is in _MAPMODES, which BGR;* modes are not, and
• frombytes() calls new(), which contains a deprecation.

The suggestion is not to deprecate the rawmodes, only the modes.