Python contributing instructions are out of date

[anilbey]

Context

Overview of the issue

The CONTRIBUTING.md file is out of date.

https://github.com/neuronsimulator/nrn/blob/17be061e031701736cb03a5088b7ddf253a069a4/CONTRIBUTING.md

It says Python unit tests are located under test/pynrn and use the [pytest](https://docs.pytest.org/) framework however there is no test/pynrn directory.

How shall we run the Python tests?

[pramodk]

Just to add here, ctest is our default way to run neuron tests as we do under CI setup.

@kbvw: just assining this to you as something quick to do, when time permits!