fix: synchronize requirements.txt, which is what Travis is using

oesteban · oesteban · commit 39522fc3355c · 2019-07-31T17:27:47.000-07:00
diff --git a/requirements.txt b/requirements.txt
@@ -8,6 +8,7 @@ numpy>=1.9.0
 packaging
 pathlib2
 prov>=1.5.2
+lxml<4.4.0 ; python_version == "3.4"
 neurdflib
 pydot>=1.2.3
 pydotplus
