doc: recommend running tests with pytest

The nosetests framework is effectively dead and earlier work has made the
tests run with pytest, which is today's de facto test runner in Python.

Author: mweinelt

CONTRIBUTING.md

@@ -93,9 +93,9 @@ One branch per bug/feature. Don't stack PR on top of other PR branches!
 
 ### Testing
 
-Testing can be done by using [Nose](http://nose.readthedocs.io/). 
-As described in Nose documentation, run `nosetests3` in the top
-level of the project directory.
+Testing can be done by using [pytest](http://pytest.org).
+As described in the [pyest documentation](https://docs.pytest.org/en/stable/how-to/usage.html),
+run `pytest` in the top level of the project directory.
 
 Additionally, the project is [configured to use the free Travis CI]
 (https://travis-ci.org/chfoo/wpull).