Bump certifi from 2019.9.11 to 2019.11.28

Bumps [certifi](https://github.com/certifi/python-certifi) from 2019.9.11 to 2019.11.28.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](certifi/python-certifi@2019.09.11...2019.11.28)

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

requirements.txt

@@ -1,7 +1,7 @@
 algoliasearch==2.1.0
 attrs==19.3.0
 bcrypt==3.1.7
-certifi==2019.9.11
+certifi==2019.11.28
 cffi==1.13.2
 chardet==3.0.4
 colander==1.7.0