using sphinx 1.8.0 breaks our doc test CI job.
Typical error:
```
Exception occurred:
File
"/home/jenkins-build/build/workspace/ceph-ansible-docs-pull-requests/docs/.tox/docs/lib/python2.7/site-packages/sphinx/highlighting.py", line 26, in <module>
from sphinx.ext import doctest
SyntaxError: unqualified exec is not allowed in function 'run' it contains a nested function with free variables (doctest.py, line 97)
```
See: https://github.com/sphinx-doc/sphinx/issues/5417
Pinning to 1.7.9 to fix our CI.
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>