Stop building docs with (test-)requirements.txt

Change Iba797243d2a137b551223165a1af1a8676bcea02 was a bit
overzealous in using {[testenv]deps} and changed the docs
tox target to also install requirements.txt and
test-requirements.txt which means for docs builds we're
installing things like psycopg2 which we shouldn't be doing
if we don't have the correct native packages installed to
build those types of dependencies.

Change-Id: Ib718911596b93ec6ec7e899210300d2f0d9572ed
Closes-Bug: #1849870

tox.ini

@@ -160,8 +160,10 @@ commands =
 [testenv:docs]
 description =
   Build main documentation.
+# Note that we don't use {[testenv]deps} for deps here because we don't want
+# to install (test-)requirements.txt for docs.
 deps =
-  {[testenv]deps}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build/html doc/build/doctrees