Log Message:
py-w3lib: updated to 1.21.0

1.21.0:
- Add the encoding and path_encoding parameters to
  :func:w3lib.url.safe_download_url
- :func:w3lib.url.safe_url_string now also removes tabs and new lines
- :func:w3lib.html.remove_comments now also removes truncated comments
- :func:w3lib.html.remove_tags_with_content no longer removes tags which
  start with the same text as one of the specified tags
- Recommend pytest instead of nose to run tests