docs: fix manpage generation with python3

Python2 manpage generation seems unaffected.

Warnings occur when generating with python3:

.../docs/source/configuration/segments/vim.rst:27: WARNING: error while
formatting arguments for
powerline.segments.vim.plugin.tagbar.current_tag: decoding str is not
supported

docs/source/powerline_automan.py

@@ -7,6 +7,8 @@ import codecs
 
 from collections import namedtuple
 
+from functools import reduce
+
 from docutils.parsers.rst import Directive
 from docutils.parsers.rst.directives import unchanged_required
 from docutils import nodes