Update version of PyYAML

Change-Id: Ibe3a0203e90e90cdbb54808abc8e46832702f498
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
(cherry picked from commit 32f92ce2d3df9e66686cebe499a51c0964c23acb)
diff --git a/requirements.in b/requirements.in
index cc55f0d..1dd93f6 100644
--- a/requirements.in
+++ b/requirements.in
@@ -34,6 +34,6 @@
 python-novaclient
 python-cinderclient==8.3.0
 pyvcloud
-pyyaml==5.4.1 # dependency comes from common
+pyyaml>6
 requests
 six