Enforce Python venv usage, install meson and ninja via venv pip

[lhearachel]

Ubuntu 24.04 ships with Python 3.12 as the default system installation, which enforces PEP 668. The present installation instructions for the project do not wholly account for this nor make it easy for non-savvy users to decide on how to resolve the externally-managed-environment error. The easiest way to resolve it which many users will come across is to run the pip install command with the --break-system-packages switch, which is ill-advised for a whole host of reasons.

The correct path forward, instead, is to enforce usage of meson and ninja via a virtual environment, per recommendations in PEP 668. This change-set:

1. Adds a new target which will create the virtual environment, defaulting to .venv,
2. Creates dependencies for the .venv on the targets which build mwrap and $(BUILD)/build.ninja, and
3. Wraps all invocations of meson and ninja in an implicit venv activate.

As a secondary benefit to this procedure, we now have the full breadth of PyPI at our disposal, should we wish to make use of external dependencies from it in our Python-based tooling. However, this must be met with scrutiny, and we should establish clear guidelines on what dependencies will be permitted as extensions to Python's standard library.

[lhearachel]

I would like for us to evaluate pinning this version requirement to a more recent minor-version, now that we are enforcing installation via a venv managed by the Makefile. I think it may also make sense to have a venv-update target which performs a pip install --update on the requirements.txt list.

It may even make sense for us to fold that into the existing make update target. Thoughts?