Bump certifi from 2018.10.15 to 2019.3.9

Bumps [certifi](https://github.com/certifi/python-certifi) from 2018.10.15 to 2019.3.9.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](certifi/python-certifi@2018.10.15...2019.03.09)

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

Author: dependabot-support

requirements.txt

@@ -1,6 +1,6 @@
 atomicwrites==1.2.1
 attrs==18.2.0
-certifi==2018.10.15
+certifi==2019.3.9
 chardet==3.0.4
 idna==2.7
 more-itertools==4.3.0