Bump python-dateutil from 1.5 to 2.8.1

Bumps [python-dateutil](https://github.com/dateutil/dateutil) from 1.5 to 2.8.1.
- [Release notes](https://github.com/dateutil/dateutil/releases)
- [Changelog](https://github.com/dateutil/dateutil/blob/master/NEWS)
- [Commits](https://github.com/dateutil/dateutil/commits/2.8.1)

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

requirements.txt

@@ -23,7 +23,7 @@ ptyprocess==0.5.1
 py==1.10.0
 pyOpenSSL==20.0.1
 pyasn1==0.1.9
-python-dateutil==1.5
+python-dateutil==2.8.1
 requests==2.22.0
 schema==0.7.4
 simplegeneric==0.8.1