dev-python/botocore: allow recent Sphinx versions

dev-python/botocore: allow recent Sphinx versions

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

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

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

Link: https://github.com/boto/botocore/pull/1306
Acked-by: Alex Brandt <alunduil@gentoo.org>
Package-Manager: Portage-2.3.12, Repoman-2.3.3