Update python-dateutil requirement from >=2.5.3 to >=2.9.0.post0

Updates the requirements on [python-dateutil](https://github.com/dateutil/dateutil) to permit the latest version.
- [Release notes](https://github.com/dateutil/dateutil/releases)
- [Changelog](https://github.com/dateutil/dateutil/blob/master/NEWS)
- [Commits](dateutil/dateutil@2.5.3...2.9.0.post0)

---
updated-dependencies:
- dependency-name: python-dateutil
  dependency-version: 2.9.0.post0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 certifi>=14.05.14 # MPL
 six>=1.9.0  # MIT
-python-dateutil>=2.5.3  # BSD
+python-dateutil>=2.9.0.post0  # BSD
 setuptools>=21.0.0  # PSF/ZPL
 pyyaml>=5.4.1  # MIT
 websocket-client>=0.32.0,!=0.40.0,!=0.41.*,!=0.42.* # LGPLv2+