New test command, update readme

[merretbuurman]

Currently README recommends running the tests via python setup.py test , but this gives a deprecated warning (in python 3.6.8), so we should recommend a different command. See error message below.

(venv3) [m@localhost PYHANDLE]$ python setup.py test
/home/merret/work/repos_and_scripts/pyhandle/PYHANDLE/pyhandle/venv3/lib/python3.6/site-packages/setuptools/dist.py:493: UserWarning: Normalizing '1.0.5-dev' to '1.0.5.dev0'
  warnings.warn(tmpl.format(**locals()))
running test
WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.
/home/merret/work/repos_and_scripts/pyhandle/PYHANDLE/pyhandle/venv3/lib/python3.6/site-packages/setuptools/installer.py:30: SetuptoolsDeprecationWarning: setuptools.installer is deprecated. Requirements should be satisfied by a PEP 517 installer.
  SetuptoolsDeprecationWarning,

[merretbuurman]

Note: The Dockerfiles that jenkins runs for testing use a different method using nose library, but nose is no longer being actively maintained (https://nose.readthedocs.io/en/latest/#note-to-users).

[merretbuurman]

Let's look into pytest and possible use pytest from now on!