Add setuptools to requirements-dev.txt

[GideonBear]

There's a test failure when setuptools is not installed in the current environment:

---------------------------------------------------------------------------------------- Captured stderr call ----------------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/pytest-of-gideon/pytest-30/test_python_integration0/gits/setup.py", line 3, in <module>
    from setuptools import setup
ModuleNotFoundError: No module named 'setuptools'
[...]
FAILED tests/make_repo_test.py::test_python_integration - subprocess.CalledProcessError: Command '('/home/gideon/projects/pre-commit-mirror-maker-2/.venv/bin/python3', 'setup.py', 'egg_info')' returned non-zero exit status 1.

uv venv does not install setuptools into the environment by default. Adding setuptools to requirements-dev.txt makes the requirement explicit.

[GideonBear]

When using tox (tox -epy), I could not get it to work without this patch (by installing setuptools manually), presumably because it creates a dedicated venv? But it creates the venv using uv because it's installed? I'm not sure.

In any case I think the dependency on setuptools should be explicit.