python 3.10 compatibility : remove old buffer api

[totaam]

See python/cpython#21117

Almost all the callers go via https://github.com/Xpra-org/xpra/blob/master/xpra/buffers/buffers.c

The only caller for object_as_write_buffer is unpremultiply_argb_in_place which is only used for cursors and window icons, so we could just copy the data if that makes it easier to move away from the old buffer api.

The read-only object_as_buffer variant has many callers:

$ grep -r object_as_buffer . | egrep -v "import|cdef|_object_as_buffer" | wc -l
41

[totaam]

This was worth doing, found a major bug in the webp encoder.