Fixes #1074: Require ncclient 0.5.3 (Python 3 fix)

requirements.txt

@@ -10,7 +10,7 @@ djangorestframework>=3.5.0
 graphviz>=0.4.10
 Markdown>=2.6.7
 natsort>=5.0.0
-ncclient==0.5.2
+ncclient==0.5.3
 netaddr==0.7.18
 paramiko>=2.0.0
 psycopg2>=2.6.1