Bump certifi from 2019.11.28 to 2020.6.20

Bumps [certifi](https://github.com/certifi/python-certifi) from 2019.11.28 to 2020.6.20.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](certifi/python-certifi@2019.11.28...2020.06.20)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -3,7 +3,7 @@ async-timeout==3.0.1
 attrs==20.2.0
 beautifulsoup4==4.9.1
 bs4==0.0.1
-certifi==2019.11.28
+certifi==2020.6.20
 chardet==3.0.4
 discord.py==1.4.1
 idna==2.10