python3-docutils: upgrade 0.18 -> 0.18.1

Changelog:
docutils/nodes.py
  Node.traverse() returns a list again to restore backwards compatibility. Fixes bug #431.
  New method Node.findall(): like Node.traverse() but returns an iterator. Obsoletes Node.traverse().

docutils/utils/__init__.py:
  Fix behaviour of get_stylesheet_list(): do not look up stylesheets given as "stylesheet" setting. Cf. bug #434.

docutils/writers/_html_base.py:
  Fix handling of footnote_backlinks==False (report Alan G Isaac).

docutils/writers/html5_polyglot/math.css:
  Fix typo (bug #432).

docutils/writers/odf_odt/__init__.py:
  Fix spurious output with Windows (bug #350).

test/test_error_reporting.py:
  Fix a false positive (bug #434).

License-Update:
 year updated to 2021
 update url of elyxer
 add url of BSD 3-Clause License
 change "2-Clause BSD license" to "BSD 2-Clause License"

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>