dev-python/flask-cors: Drop ROOT and use stricter python-docs dependencies

dev-python/flask-cors: Drop ROOT and use stricter python-docs dependencies

We're not supposed to use ROOT in ebuild functions (see the discussion
in f3893ec9, dev-python/flask-cors/flask-cors-9999.ebuild: Use local
intersphinx, 2014-09-02).  I had been using it to build against the
objects.inv from that ROOT, but we're actually supposed to build
against the local (non-ROOT) copy [1]:

  ROOT isn't used for library dependencies. When building, libraries,
  headers etc. on / have to be used. Cross-compiling etc is done by
  configuring the toolchain correctly, not by hacking ebuilds in a way
  that breaks binary packages.

I'm also using equery to list the python-docs files (on /), to avoid
hard-coding /usr/share/doc"/python-docs-*/html.  equery knows what was
installed, and I really don't care where it is, so long as I can find
the objects.inv (and so long as that's the directory with the HTML
docs too).

Finally, the python-docs dependencies make sure we accept any of the
python-docs versions that install objects.inv [2].  Ian Delaney bumped
versions to -r1 so we could determine if objects.inv was installed,
and Ian Stakenvicius pointed out the slotted approach to the disjoint
dependency.

[1]: https://bugs.gentoo.org/show_bug.cgi?id=258125#c0
[2]: https://bugs.gentoo.org/show_bug.cgi?id=521768

Thanks-to: Ian Delaney <idella4@gentoo.org>
Thanks-to: Ian Stakenvicius <axs@gentoo.org>