sphinx.eclass: Make local intersphinx links easier to setup

sphinx.eclass: Make local intersphinx links easier to setup

See:
https://bugs.gentoo.org/show_bug.cgi?id=522794
  dev-python/python-docs: Install files in a more predictable location
https://bugs.gentoo.org/show_bug.cgi?id=526186
  distutils-r1 / python-utils-r1: need better way of installing Sphinx docs
https://bugs.gentoo.org/show_bug.cgi?id=521768
  dev-python/python-docs-3.4.0 doesn't install objects.inv for intersphinx
https://bugs.gentoo.org/show_bug.cgi?id=517478
  dev-python/flask-cors - A Flask extension adding a decorator for CORS support