Bump pyyaml from 4.2b1 to 5.3

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 4.2b1 to 5.3.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/commits/5.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -1,4 +1,4 @@
-PyYaml==4.2b1
+PyYaml==5.3
 azure-common==1.1.24
 azure-mgmt-dns==3.0.0
 boto3==1.11.0