pyln: actually specify jsonrpc in requests

As per the spec (https://www.jsonrpc.org/specification#request_object)

```
A rpc call is represented by sending a Request object to a Server. The Request object has the following members:

jsonrpc
    A String specifying the version of the JSON-RPC protocol. MUST be exactly "2.0".
```

Changelog-fixed: pyln now includes the "jsonrpc" field to jsonrpc2 requests

contrib/pyln-client/pyln/client/lightning.py

@@ -285,6 +285,7 @@ class UnixDomainSocketRpc(object):
         # FIXME: we open a new socket for every readobj call...
         sock = UnixSocket(self.socket_path)
         self._writeobj(sock, {
+            "jsonrpc": "2.0",
             "method": method,
             "params": payload,
             "id": self.next_id,