Generates a bytes version of a Message object tree.
Functionally identical to the base Generator except that the output is
bytes and not string. When surrogates were used in the input to encode
bytes, these are decoded back to bytes for output. If the policy has
cte_type set to 7bit, then the message is transformed such that the
non-ASCII bytes are properly content transfer encoded, using the charset
unknown-8bit.
The outfp object must accept bytes in its write method.
Encode s using the ASCII codec and the surrogateescape error handler, and pass it to the write method of the outfp passed to the BytesGenerator’s constructor.