docs(contributing): require pip>=25.1 for dependency groups

[tr1plejj]

Problem

The third setup step in docs/contributing.rst fails on a new virtualenv:

$ python3 -m venv .venv
$ source .venv/bin/activate
$ pip install --group dev

Usage:
  pip install [options] <requirement specifier> [package-index-options] ...
  ...
no such option: --group

$ python3 --version
Python 3.12.3
$ pip --version
pip 24.0 from /home/***/dishka/.venv/lib/python3.12/site-packages/pip (python 3.12)

Creating the virtualenv with --upgrade-deps makes it work:

$ python3 -m venv --upgrade-deps .venv
$ pip --version
pip 26.2 from /home/***/dishka/.venv/lib/python3.12/site-packages/pip (python 3.12)

$ pip install --group dev
...
Successfully installed ... mypy-2.1.0 nox-2026.4.10 ruff-0.15.15 uv-0.11.19 ...

Why

The --group option was added in pip 25.1 (2025-04-26):
https://pip.pypa.io/en/stable/news/#v25-1

Add a --group option which allows installation from PEP 735 Dependency Groups.

The --upgrade-deps flag upgrades pip when the virtualenv is created: https://docs.python.org/3/library/venv.html

--upgrade-deps: Upgrade core dependencies (pip) to the latest version in PyPI.

A newly created virtualenv may have an older pip, so the very first setup
command fails for new contributors as it did in my case.

[Niccolum]

maybe just --upgrade-deps flag add here? Starts with 3.9

[tr1plejj]

Yes, just checked and it worked, looks cleaner, thanks. Also changed PR body

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[chirizxc]

LGTM