Bump boto3 from 1.14.52 to 1.15.9

Bumps [boto3](https://github.com/boto/boto3) from 1.14.52 to 1.15.9.
- [Release notes](https://github.com/boto/boto3/releases)
- [Changelog](https://github.com/boto/boto3/blob/develop/CHANGELOG.rst)
- [Commits](https://github.com/boto/boto3/compare/1.14.52...1.15.9)

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

requirements.txt

@@ -1,7 +1,7 @@
 PyYaml==5.3.1
 azure-common==1.1.25
 azure-mgmt-dns==3.0.0
-boto3==1.14.52
+boto3==1.15.9
 botocore==1.18.9
 dnspython==1.16.0
 docutils==0.16