Add dependency on yamllint

dib-lint requires yamllint now, so we need to depend on it.

Change-Id: Iccd2cdbb04bd9b429d8d19b31ff3b10cdd568e15

releasenotes/notes/yamllint-c07fd31780ec8d10.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    A dependency on ``yamllint`` was added because it is now
+    required by dib-lint.

requirements.txt

@@ -8,5 +8,6 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 PyYAML>=3.12 # MIT
 six>=1.10.0 # MIT
 stevedore>=1.20.0 # Apache-2.0
-# NOTE(ianw) in here because dib-lint uses flake8
+# NOTE(ianw) in here because dib-lint uses flake8 and yamllint
 flake8<4.0.0,>=3.6.0 # MIT
+yamllint>=1.23.0

test-requirements.txt

@@ -12,5 +12,3 @@ testrepository>=0.0.18 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
 
 coverage!=4.4,>=4.0 # Apache-2.0
-
-yamllint>=1.23.0