Add sphinx constraint for rstcheck compatibility.

test/runner/requirements/constraints.txt

@@ -2,6 +2,7 @@ coverage >= 4.2, != 4.3.2 # features in 4.2+ required, avoid known bug in 4.3.2
 cryptography < 2.2 ; python_version < '2.7' # cryptography 2.2 drops support for python 2.6
 pywinrm >= 0.3.0 # message encryption support
 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
 pycrypto >= 2.6 # Need features found in 2.6 and greater