This commit removes Six as a dependency for Pelican, replacing the
relevant aliases with the proper Python 3 imports. It also removes
references to Python 2 logic that did not require Six.
Addresses https://github.com/getpelican/pelican/issues/2523
Note: @avaris made a good point that there is no need to replace the existing
module runner for pelican quickstart or the other tools as this can be run
via:
> python -m pelican.tools.pelican_quickstart
