Pin pygments to 2.3.x

Pygments release 2.4.0 which added a python requires that excludes all
versions of Python < 3.5. The LFCI's default 3 version is 3.4 so causes
build failure.

Change-Id: Ie1e4281efee4461f61cdffa3791c22cd5fbf54ec
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>
diff --git a/tox.ini b/tox.ini
index e33d587..5ff30f8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,6 +8,7 @@
 deps =
     coala==0.11
     coala-bears==0.11
+    pygments~=2.3.1
     nodeenv~=1.3.0
 commands =
     nodeenv -p