testing/py3-pelican: use github URL and drop unnecessary dep on py3-six

- It's easier to download files from github than from https://pypi.org/project/pelican/#files
- Since Pelican is now python3 and isn't using six anymore, we can drop it from
  the dependencies.
7 jobs for !19172 with drop_six in 1 minute and 40 seconds (queued for 1 second)
merge request