Bump certifi from 2019.11.28 to 2020.4.5.1

Bumps [certifi](https://github.com/certifi/python-certifi) from 2019.11.28 to 2020.4.5.1.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](https://github.com/certifi/python-certifi/commits)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -1,6 +1,6 @@
 atomicwrites==1.3.0
 attrs==19.3.0
-certifi==2019.11.28
+certifi==2020.4.5.1
 chardet==3.0.4
 codecov==2.0.22
 contextlib2==0.6.0.post1