Bump Sphinx lower constraints to 1.8.0

We already bumped Sphinx to 1.8.0 in doc reqs

refer to https://review.opendev.org/#/c/688371/

Change-Id: Ifd46ab6b2043392aa7878a11b0fe186f5ee4c4ce

lower-constraints.txt

@@ -89,7 +89,7 @@ simplejson==3.13.2
 six==1.10.0
 smmap2==2.0.3
 snowballstemmer==1.2.1
-Sphinx==1.6.2
+Sphinx==1.8.0
 sphinxcontrib-websupport==1.0.1
 sphinxcontrib-svg2pdfconverter==0.1.0
 stevedore==1.28.0