In the documentation, both `SOCIAL` and `LINKS` are described as a list
of tuples, but all examples show them as tuples of tuples. It makes the
most sense for them to be lists, so the examples should match the docs
going forward.
Similarly, `LOCALE` is described as a list, but the examples show it as
a tuple. Likewise, the examples should be lists, in line with the
description.
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.
