Install future before running sphinx

Fix Sphinx warning about lack of module named 'future' by making the
installation of future a dependency of sphinx in tox.ini

Reviewed By: danilaml

Differential Revision: https://reviews.llvm.org/D95069

Author: Thomas Preud'homme

tox.ini

@@ -38,6 +38,7 @@ skip_install = true
 deps =
     sphinx<2.0.0
     sphinx_bootstrap_theme
+    future
 
 commands =
     make -C {toxinidir}/docs/ html