Handle /release/ directory

[JulienPalard]

docsbuild-scripts could manage the /release/ directory too, this directory is currently manually handled by RMs, see PEP 101.

[JulienPalard]

www.python.org/ftp/python/doc/

• Maintained manually by release managers.
• Contains a current/ directory pointing to the current stable release.
• Each directory contains an epub file and tarballs and zipfiles of: pdf-a4, pdf-letter, epub, html, texinfo and text.

www.python.org/doc/versions/

Contains links to http://docs.python.org/release/x.x.x/ for each release, updated manually by release managers.

Download pages in docs.python.org/releases/

In those download pages, download links looks like:

https://docs.python.org/ftp/python/doc/3.6.2/python-3.6.2-docs-pdf-letter.zip

which are just redirections to:

https://www.python.org/ftp/python/doc/3.6.2/python-3.6.2-docs-pdf-letter.zip

This is the result of the daily flag:

{% if daily is defined %}
   {% set dlbase = pathto('archives', 1) %}
{% else %}
   {% set dlbase = 'https://docs.python.org/ftp/python/doc/' + release %}
{% endif %}

[AA-Turner]

/release/ is now handled automatically by run_release.py.

A