Update python-dateutil requirement from >=2.7 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@2.7.0...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

@@ -1,4 +1,4 @@
-python-dateutil>=2.7
+python-dateutil>=2.9.0.post0
 inflection>=0.3
 unidecode>=1.0
 Jinja2>=2.10