Remove obsolete dependency

Python < 3.9 is no longer supported.

Author: WhyNotHugo

pyproject.toml

@@ -35,7 +35,6 @@ dependencies = [
   "django_renderpdf>=3.0.0,<5.0.0",
   "lxml>=3.4.4",
   "pyopenssl>=16.2.0",
-  'backports.zoneinfo;python_version<"3.9"',
   "setuptools-git>=1.1",
   "wheel>=0.24.0",
   "zeep>=1.1.0,<5.0.0",