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.

* Convert FEED settings from `%s` to `{slug}` redux

Closes #2106, Closes #2383

It's now present in both RSS and Atom feeds.

Removed `PDF_GENERATOR=False`

It might give the impression that the PDF generation is still part of the core.

- DEFUALT_METADATA, tuple -> dictionary
- OUTPUT_RETENTION, tuple -> list
- JINJA_FILTERS, list -> dictionary
- DIRECT_TEMPLATES, tuple -> list

On OSX, if LC_TIME and LC_CTYPE differs the output of strftime is not properly decoded
in Python 3. This makes sure that the 'utils.DateFormatter' and the related Jinja filter
'strftime' set the same value for LC_TIME and LC_CTYPE while formatting.

Also, '%a' is removed from DEFAULT_DATE_FORMAT in 'custom_locale' tests. OSX and *nix have
different conversions for '%a' ('Jeu' vs 'jeu.') and there is not a feasible way to handle
the difference for tests.

reverts getpelican/pelican@ddcccfeaa952d2e1e24ceac94e5d66c73b57c01b

If one used a locale that made use of unicode characters (like fr_FR.UTF-8)
the files on disk would be in correct locale while links would be to C.

Uses a SafeDatetime class that works with unicode format strigns
by using custom strftime to prevent ascii decoding errors with Python2.

Also added unicode decoding for the calendar module to fix period
archives.

This allows for templates using {% if article.modified %}
instead of {% if article.modified != article.published %} .