docsrc: update for Sphinx 3.x
Sphinx 1.8 deprecates sphinx.util.osutil.ustrftime and
https://github.com/sphinx-doc/sphinx/commit/61098a0ae2e696a804 removes
the method.
- update docsrc/exts/sphinxlocal/writers/manpage.py to compile with Sphinx-doc 3.2.1.