Move setuptools to testing requirements

It was defined in `testenv.deps` for tox, making it a strict testing requirement. The version requirement is copied from `pyproject.toml`.

Author: ulgens

requirements/requirements-testing.txt

@@ -6,3 +6,4 @@ importlib-metadata<5.0
 # temporary pin of attrs
 attrs==22.1.0
 pytz  # Remove when dropping support for Django<5.0
+setuptools>=77.0.3

tox.ini

@@ -23,7 +23,6 @@ deps =
         djangomain: https://github.com/django/django/archive/main.tar.gz
         -rrequirements/requirements-testing.txt
         -rrequirements/requirements-optionals.txt
-        setuptools
 
 [testenv:base]
 ; Ensure optional dependencies are not required