cbbackup can't backup mutations with rev over 256

Raised in the forums.
Problem
cbbackup fails when the rev of a mutation is over 256

  File "couchbase-cli/pump_dcp.py", line 413, in provide_dcp_batch_actual

    msg = (cmd, vbucket_id, key, flg, exp, cas, bytes([rev_seqno]), val, seqno, dtype, metalen,

ValueError: bytes must be in range(0, 256)

Steps to reproduce

1. Create the default bucket
2. Use pillowfight to populate the bucket
3. Leave pillowfight running for enough time so each document is update more than 256 times.
4. run cbbackup

   ./cbbackup http://localhost:8091 ~/backup -u Administrator -p password -b default -m full

Expectation
For cbbackup to be able to backup the data successfully.