Drop direct reference to `ez_setup.py`/`setuptools` from venv's EnvBuilder example

[pradyunsg]

From #95299:

https://docs.python.org/3.12/library/venv.html#an-example-of-extending-envbuilder is the final piece of work here -- that example needs to be updated to use something more modern or otherwise behave differently to showcase extending behaviours of the class.

This is relying a deprecated mechanism to install setuptools, which it should move away from.