Switch to python3 sphinx.

debian/changelog

@@ -4,6 +4,7 @@ cmake (3.9.3-1) UNRELEASED; urgency=medium
   * Increase test timeout to 2000s.
     - Hopefully this gives slow buildds enough time to run the Qt5Autogen test.
       (Closes: #876378)
+  * Switch to python3 sphinx.
 
  -- Felix Geyer <fgeyer@debian.org>  Thu, 21 Sep 2017 19:04:21 +0200
 

debian/control

@@ -16,7 +16,7 @@ Build-Depends: debhelper (>= 9.20160114~),
                librhash-dev,
                libuv1-dev,
                procps [!hurd-any],
-               python-sphinx,
+               python3-sphinx,
                qtbase5-dev <!stage1>,
                zlib1g-dev
 Standards-Version: 3.9.6