> The patch contains a special case for writing only one bytes object.
> This is very unlikely case.

The patch only modify a few functions to make them use the new _PyBytesWriter API. Other functions can use it.

A few examples:

 - PyBytes_FromObject()
 - binascii: binascii_rledecode_hqx()
 - bz2, lzma and zlib modules
 - marshal and pickle modules
 - datetime.datetime.strftime()
 - Python/compile.c: assemble_lnotab()
 - more Unicode decoders

But I agree that the readonly "hack" can be removed from _PyBytesWriter API since the bytes type has no format method (no bytes%args nor bytes.format(args)).