[enh] utils/lib.sh - commands pyenv, pyenv.drop pyenv.(un)install

Implement a boilerplate to manage performance optimized virtualenv builds.
Shell scripts can use (e.g.) 'pyenv.cmd' to execute command in the virtualenv
without having to worry about whether and how the environment is provided. ::

  pyenv.cmd which python
  ..../local/py3/bin/python

  pyenv.cmd which pip
  ..../local/py3/bin/pip

If pyenv.cmd released multiple times the installation will only rebuild if the
function 'pyenv.OK' fails.  Function 'pyenv.OK' make some test to validate that
the virtualenv exists and works as expected.  The check also fails if
requirements listed requirements-dev.txt and requirements.txt has been edited.
Among these tests 'pyenv.OK' calls 'pyenv.check' which implements a python
script that validate the python installation.  Here is an example how a
'pyenv.check' implementation could look like::

    pyenv.check() {
       cat  <<EOF
    import yaml
    print('import yaml --> OK')
    EOF
    }

Signed-off-by: Markus Heiser <markus@darmarit.de>