update pyyaml version

 - known vulnerability in version <4.1 [1]

[1] yaml/pyyaml#74

Author: wolverineav

requirements.txt

@@ -4,5 +4,5 @@
 pbr>=0.11,<2.0
 subprocess32>=3.2.6,<3.2.9
 futures>=3.0.2,<3.0.5
-pyyaml>=3.11,<3.15
+pyyaml>=4.1,<4.3
 netaddr>=0.7.18,<0.8