Bump dnspython from 1.15.0 to 1.16.0

Bumps [dnspython](https://github.com/rthalley/dnspython) from 1.15.0 to 1.16.0.
- [Release notes](https://github.com/rthalley/dnspython/releases)
- [Changelog](https://github.com/rthalley/dnspython/blob/master/doc/whatsnew.rst)
- [Commits](https://github.com/rthalley/dnspython/compare/v1.15.0...v1.16.0)

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

requirements.txt

@@ -3,7 +3,7 @@ azure-common==1.1.24
 azure-mgmt-dns==3.0.0
 boto3==1.11.0
 botocore==1.14.0
-dnspython==1.15.0
+dnspython==1.16.0
 docutils==0.15.2
 dyn==1.8.1
 edgegrid-python==1.1.1