Add sphinx constraint for rstcheck compatibility.

(cherry picked from commit 7507bc9e19)

test/runner/requirements/constraints.txt

@@ -6,6 +6,7 @@ astroid == 1.5.3 ; python_version >= '3.5' # newer versions of astroid require n
 pylint == 1.7.4 ; python_version >= '3.5' # versions before 1.7.1 hang or fail to install on python 3.x
 pylint == 1.6.5 ; python_version <= '2.7' # versions after 1.6.5 hang or fail during test on python 2.x
 sphinx < 1.6 ; python_version < '2.7' # sphinx 1.6 and later require python 2.7 or later
+sphinx < 1.8 ; python_version >= '2.7' # sphinx 1.8 and later are currently incompatible with rstcheck 3.3
 wheel < 0.30.0 ; python_version < '2.7' # wheel 0.30.0 and later require python 2.7 or later
 yamllint != 1.8.0 ; python_version < '2.7' # yamllint 1.8.0 requires python 2.7+ while earlier/later versions do not
 isort < 4.2.8 # 4.2.8 changes import sort order requirements which breaks previously passing pylint tests