Bump certifi from 2020.4.5.2 to 2020.6.20

Bumps [certifi](https://github.com/certifi/python-certifi) from 2020.4.5.2 to 2020.6.20.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](certifi/python-certifi@2020.04.05.2...2020.06.20)

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

requirements.txt

@@ -1,7 +1,7 @@
 asn1crypto==1.3.0
 boto3==1.14.7
 bcrypt==3.1.7
-certifi==2020.4.5.2
+certifi==2020.6.20
 cffi==1.14.0
 chardet==3.0.4
 colander==1.7.0