Install or update the pyenv python environment.

    Checkout or update the pyenv repo at ~/.pyenv and enable the pyenv.
    Pyenv wird also als Github-Repo "installiert" unter ~/.pyenv

    More info:
     * https://github.com/yyuu/pyenv
     * https://github.com/yyuu/pyenv/wiki/Common-build-problems#requirements
    Tutorial:
     * http://amaral-lab.org/resources/guides/pyenv-tutorial

def pyenv():
    '''Install or update the pyenv python environment.

    Checkout or update the pyenv repo at ~/.pyenv and enable the pyenv.
    Pyenv wird also als Github-Repo "installiert" unter ~/.pyenv

    More info:
     * https://github.com/yyuu/pyenv
     * https://github.com/yyuu/pyenv/wiki/Common-build-problems#requirements
    Tutorial:
     * http://amaral-lab.org/resources/guides/pyenv-tutorial
    '''
    install_packages([
        'make',
        'build-essential',
        'libssl-dev',
        'zlib1g-dev',
        'libbz2-dev',
        'libreadline-dev',
        'libsqlite3-dev',
        'wget',
        'curl',
        'llvm',
        'libncurses5-dev',
        'libncursesw5-dev',
    ])
    if exists('~/.pyenv'):
        run('cd ~/.pyenv  &&  git pull')
        run('~/.pyenv/bin/pyenv update')
    else:
        run('curl -L https://raw.githubusercontent.com/yyuu/pyenv-installer/'
            'master/bin/pyenv-installer | bash')

    # add pyenv to $PATH and set up pyenv init
    bash_snippet = '~/.bashrc_pyenv'
    install_file_legacy(path=bash_snippet)
    prefix = flo('if [ -f {bash_snippet} ]; ')
    enabler = flo('if [ -f {bash_snippet} ]; then source {bash_snippet}; fi')
    if env.host == 'localhost':
        # FIXME: next function currently only works for localhost
        uncomment_or_update_or_append_line(filename='~/.bashrc', prefix=prefix,
                                           new_line=enabler)
    else:
        print(cyan('\nappend to ~/.bashrc:\n\n    ') + enabler)