Remove sphinx from test-requirements.txt

Documentation build environments use doc/requirements.txt where sphinx
is already present. Use doc/requirements.txt in the pep8 environment
because sphinx is required by sphinx8.

Change-Id: I3a2c9ae392745e001c5bdd01212c996e54c44f0c

test-requirements.txt

@@ -12,8 +12,5 @@ molecule<3 # MIT
 oslotest>=1.10.0 # Apache-2.0
 # paramiko 2.5.0 makes the kayobe-tox-molecule job fail.
 paramiko<2.5.0
-# NOTE(mgoddard): This is required by tools/sphinx8.
-sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
-sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4'  # BSD
 stestr # Apache-2.0
 yamllint # GPLv3

tox.ini

@@ -25,6 +25,12 @@ commands = stestr run {posargs}
 
 [testenv:pep8]
 basepython = python3
+# sphinx8 needs the sphinx package which is required via doc/requirements.txt
+deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/requirements.txt
+    -r{toxinidir}/doc/requirements.txt
+    -r{toxinidir}/test-requirements.txt
 commands =
   {toxinidir}/tools/run-bashate.sh
   flake8 {posargs} kayobe