Bump nested-lookup from 0.2.18 to 0.2.21

Bumps [nested-lookup](https://github.com/russellballestrini/nested-lookup) from 0.2.18 to 0.2.21.
- [Release notes](https://github.com/russellballestrini/nested-lookup/releases)
- [Commits](https://github.com/russellballestrini/nested-lookup/commits)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -2,7 +2,7 @@ beautifulsoup4==4.8.1
 elasticsearch==5.5.3
 flask-cors==3.0.8
 gunicorn==19.10.0
-nested-lookup==0.2.18
+nested-lookup==0.2.21
 pylint==2.3.1
 python-memcached==1.59
 requests-aws4auth==0.9