Clarify the python version for installing Django by local copy.

[chengjackjelly]

When I try to follow the instructions for [Writing your first patch for Django/Getting a copy of Django’s development version](​https://docs.djangoproject.com/en/dev/intro/contributing/#getting-a-copy-of-django-s-development-version], I encounter a problem with python version.

In the docs, we should use

python3 -m venv ~/.virtualenvs/djangodev

to create a new virtual environment by running. However, it doesn't point out the exact python3 version we should use. In the later

python -m pip install -e /path/to/your/local/clone/django/

I have a problem reported as

ERROR: Package 'Django-5.1.dev20240516091757' requires a different Python: 3.8.3 not in '>=3.10'

Later on I install and change to the python version >=3.10 to solve the problem. It would be great if the docs specify the Python Interpreter version.

[Mariusz Felisiak]

There are already many places where a specific Python 3 version is ​mentioned. It also defined in setup.cfg. I don't think it's worth adding another place that we will have to remember to keep up to date.