we also needed python3-Sphinx to build the man pages, as unlike
the autotools based build, the cmake based build does not fall
back to the pre-built man pages.
This causes build cycle. Split out the documentation building to
break the cycle. The Tumbleweed release managers preferred this
solution over a mini package.
OBS-URL: https://build.opensuse.org/package/show/graphics/tiff?expand=0&rev=185
