Update ruamel-yaml requirement from ==0.17.* to ==0.18.*

Updates the requirements on [ruamel-yaml]() to permit the latest version.

---
updated-dependencies:
- dependency-name: ruamel-yaml
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -3,7 +3,7 @@
 #
 
 # ruamel.yaml is used to read and write .yaml files.
-ruamel.yaml==0.17.*
+ruamel.yaml==0.18.*
 
 # jsonschema is used for validating cluster.yaml configurations
 jsonschema==4.*