Automatically activate/deactivate virtualenvs in `.venv` when we `cd` into their parent directory

auto-venv.sh

# auto activate virtualenvs ------------------

# I suspect this could be rewritten to just use VIRTUAL_ENV directly,
# without needing the $parentdir variable at all.

function cd() {
    builtin cd "$@"

    # If we move out of a previous parentdir, deactivate the previous venv
    if [[ -n "$parentdir" && "$PWD"/ != "$parentdir"/* ]]; then
        deactivate
        unset parentdir
    fi

    # If there is a virtualenv, activate it
    if [[ -d ".venv" && -f ".venv/bin/activate" ]]; then
        source ".venv/bin/activate"
        parentdir="$(dirname $VIRTUAL_ENV)"
    fi
}