In RPC module, "getinfo" method doesn't seem to work

Hi,

I noticed earlier today that the method `getinfo()` doesn't seem to work in the `rpc` module:

https://github.com/petertodd/python-bitcoinlib/blob/b5540e8a8a138f2a4872c34ce4223b8f4e6856d9/bitcoin/rpc.py#L513

Should `call('getinfo')` be replaced by `call('getblockchaininfo')`? (https://bitcoincore.org/en/doc/0.20.0/rpc/blockchain/getblockchaininfo/)