Override python's plain virtualenv prompt

The way python overrides the bash prompt is barbaric.

According to this SO answer the default change can be overridden by setting VIRTUAL_ENV_DISABLE_PROMPT=1.

The prompt information than then be obtained by this function:

function virtualenv_info(){
    # Get Virtual Env
    if [[ -n "$VIRTUAL_ENV" ]]; then
        # Strip out the path and just leave the env name
        venv="${VIRTUAL_ENV##*/}"
    else
        # In case you don't have one activated
        venv=''
    fi
    [[ -n "$venv" ]] && echo "(venv:$venv) "
}