fix --complete exception caused due to context manager type error

[elig0n]

This PR solves a TypeError exception that happened on python 3.12.3 when
running ddgr --complete blah on the latest git version.
It stems from the json.loads returned object (either list or dict) not accepted for the context.

  File "/usr/bin/ddgr", line 1982, in completer_fetch_completions
    with json.loads(resp.read().decode('utf-8')) as respobj:
TypeError: 'list' object does not support the context manager protocol

Feel free to comment and offer modifications.