dev-python/boto3: allow recent Sphinx versions

dev-python/boto3: allow recent Sphinx versions

Bump upper boundary of allowed version of Shpinx to 1.7 in new,
non-stabilized boto3 ebuilds.

Dependency on Sphinx has versions range. Declaration of this range comes
from requirements-docs.txt in boto3 sources, but that file hasn't been
updated since 2015. Latest Sphinx, version 1.6.5, builds boto3 docs
successfully.

Sphinx versions range bump has been suggested to boto3 maintainers, but
there's nothing preventing newer Sphinx from working on already released
versions.

Link: https://github.com/boto/boto3/pull/1337
Acked-by: Alex Brandt <alunduil@gentoo.org>
Package-Manager: Portage-2.3.12, Repoman-2.3.3