Upgrade Sphinx package

Sphinx 2.1.0 has a bug [0] that causes whitespace to be excluded in
outputted docs. This change updates Sphinx to peg any version >2.1.0.

Safety dependency vulnerability checks now will also cover the doc
requirements.txt.

[0] https://github.com/sphinx-doc/sphinx/issues/6440

Change-Id: I35f1acf4385821969ffa7c9807cf209a59c1d73e

doc/requirements.txt

@@ -1,3 +1,3 @@
 # Documentation
-sphinx>=1.6.2
+sphinx>2.1.0
 sphinx_rtd_theme==0.2.4

tox.ini

@@ -31,7 +31,8 @@ commands =
 [testenv:pep8]
 basepython = python3
 deps =
-    pipenv
+  pipenv
+  safety
 commands =
   pipenv install --dev
   bash -c {toxinidir}/tools/gate/whitespace-linter.sh
@@ -39,15 +40,18 @@ commands =
   flake8 {toxinidir}/spyglass {toxinidir}/tests
   bandit -r spyglass -n 5
   pipenv check
+  safety check -r {toxinidir}/doc/requirements.txt --bare
 whitelist_externals =
   bash
 
 [testenv:safety]
 deps =
-    pipenv
+  pipenv
+  safety
 commands =
   pipenv install --dev
   pipenv check
+  safety check -r {toxinidir}/doc/requirements.txt --full-report
 
 [testenv:bandit]
 deps =