lt: True).
    If set this to false, use `bytes()` to get content and `.reset()` to clear buffer.

:param bool use_bin_type:
    Use bin type introduced in msgpack spec 2.0 for bytes.
    It also enables str8 type for unicode. (default: True)

:param bool strict_types:
    If set to true, types will be checked to be exact. Derived classes
    from serializable types will not be serialized and will be
    treated as unsupported type and forwarded to default.
    Additionally tuples will not be serialized as lists.
    This is useful when trying to implement accurate serialization
    for python types.

:param bool datetime:
    If set to true, datetime with tzinfo is packed into Timestamp type.
    Note that the tzinfo is stripped in the timestamp.
    You can get UTC datetime with `timestamp=3` option of the Unpacker.

:param str unicode_errors:
    The error handler for encoding unicode. (default: 'strict')
    DO NOT USE THIS!!  This option is kept for very specific usage.

:param int buf_size:
    Internal buffer size. This option is used only for C implementation.
