check that ensurepip / venv package is installed before starting installation

[ievans]

On Debian, if python3-venv is not installed, bento init will fail and then subsequent runs will not work until --init clean is called. It would be preferable to check for the correct ensurepip or venv before starting the install process.

Failing command: ['/home/pi/.bento/venv/click/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']

                                                                               The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venvng up
package using the following command.․․․․․․․․․․|■■■□□□□□□□| 00:01, 🍜 Setting up

    apt-get install python3-venv․․․․․․․․․․․․․․|■■■□□□□□□□| 00:01, 🍜 Setting up

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: ['/home/pi/.bento/venv/requests/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']

bandit․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․|■■■□□□□□□□| 00:01, 🍜 Setting upThe virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.
r2c.click․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․|■■■□□□□□□□| 00:01, 🍜 Setting up
    apt-get install python3-venv
r2c.requests․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․|■■■□□□□□□□| 00:01, 🍜 Setting up
You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: ['/home/pi/.bento/venv/bandit/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']

                                                                               The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venvng up
package using the following command.

    apt-get install python3-venv

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.