py-docutils: updated to 0.16

Release 0.16:

Docutils 0.15.x is the last version supporting Python 2.6, 3.3 and 3.4.
Docutils 0.16.x supports Python 2.7 and Python >= 3.5 natively,
without the use of the ``2to3`` tool.

* reStructuredText:
  - Keep `backslash escapes`__ in the document tree. This allows, e.g.,
    escaping of author-separators in `bibliographic fields`__.
* LaTeX writer:
  - Informal titles of type "rubric" default to bold-italic and left aligned.
  - Deprecate ``\docutilsrole`` prefix for styling commands:
    use ``\DUrole`` instead.
  - Fix topic subtitle.
  - Add "latex writers" to the `config_section_dependencies`.
  - Ignore classes for `rubric` elements
    (class wrapper interferes with LaTeX formatting).

* tools/buildhtml.py
  - New option "--html-writer" allows to select "html__" (default),
    "html4" or "html5".
* docutils/io.py
  - Remove the `handle_io_errors` option from io.FileInput/Output.
* docutils/nodes.py
  - If `auto_id_prefix`_ ends with "%", this is replaced with the tag name.
* Various bugfixes and improvements (see HISTORY_).

(adam)