Session.hostkey_hash: return bytes in Python 3

As libssh2_hostkey_hash(3) returns raw binary bytes which obviously
cannot be interpreted as UTF-8, Session.hostkey_hash shall return a
bytes object in Python 3 instead of an str object.
Session.hostkey_hash: explicitly set result size

The libssh2_hostkey_hash(3) function returns raw binary bytes that may
contain non-terminating null characters if the host key contains a null
byte.