Change Sphinx version to latest (#6817)

earlephilhower · web-flow · commit b478429fe405 · 2019-11-20T08:36:04.000-07:00
Was locked at an older one which no longer works due to an issue a while
ago, but that may no longer be needed.
diff --git a/doc/requirements.txt b/doc/requirements.txt
@@ -1,6 +1,6 @@
 # Requirements file for pip
 # list of Python packages used in documentation build
-sphinx==1.7.9
+sphinx
 sphinx-rtd-theme
 breathe
 nbsphinx
