build(deps): 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] <[email protected]>

Author: dependabot-preview[bot]

requirements-pinned.txt

@@ -1,4 +1,4 @@
-certifi==2019.11.28       # via requests
+certifi==2020.4.5.1       # via requests
 cffi==1.14.0              # via cryptography, pynacl
 chardet==3.0.4            # via requests
 colorama==0.4.3           # via securesystemslib