The generate rpc call is deprecated and will be removed in v0.19. Bitcoin-cli message output when called:
The wallet generate rpc method is deprecated and will be fully removed in v0.19. To use generate in v0.18, restart bitcoind with -deprecatedrpc=generate. Clients should transition to using the node rpc method generatetoaddress'
Fix: generate(self, numblocks) in rpc.py (https://github.com/petertodd/python-bitcoinlib/blob/master/bitcoin/rpc.py#L383) needs to be updated to something along the lines of generate(self, numblocks, address).
Can make a PR if that helps.