On Python 3, `resp.content` will be bytes, not text, so this will fail with any real input:

```pytb
  File "./pysolr.py", line 318, in _send_request
    error_message = self._extract_error(resp)
  File "./pysolr.py", line 400, in _extract_error
    if resp.content and resp.content.lstrip().startswith('{'):
TypeError: startswith first arg must be bytes or a tuple of bytes, not str
```

It looks like Solr always uses UTF-8, so it should be safe to decode as UTF-8 here.