This introduces two new special commands for rb-site manage: pip

and python. These are used to execute pip and python,

respectively, using the correct version of the site.
If the site is installed with an embedded venv directory, the

executable will be run from there, ensuring the command is executing

within that virtual environment.
If the site is a traditional globally-installed site, the versioned

pythonX.Y binary for the current version of Python running rb-site

will be used. pip always uses pythonX.Y -m pip.
The python command is conceptually distinct from shell, as it's

meant to be a raw pass-through to Python, rather than an interactive

shell (Django has additional logic going into their shell and can

execute ipython or bpython).
These are intentionally not implemented as management commands, in

order to minimize any Django-related code that needs to execute in

order to work with packages or the Python interpreter.
This will be the preferred way going forward for documenting any pip

or python usage within a site. Documentation will be updated

separately to show this usage.

Tested using the pip command for installing, uninstalling, and listing

packages. Tested this with both a virtualenv install and a traditional

global install.
Tested using the python command as a shell, command runner (using -c),

and piping a script to it.