docs for nox (#3955)

* docs for nox

* add titles, notices and additional detail

* review actions

docs/iris/src/developers_guide/contributing_running_tests.rst

@@ -5,6 +5,11 @@
 Running the Tests
 *****************
 
+Using setuptools for Testing Iris
+=================================
+
+.. warning:: The `setuptools`_ ``test`` command was deprecated in `v41.5.0`_. See :ref:`using nox`.
+
 A prerequisite of running the tests is to have the Python environment
 setup.  For more information on this see :ref:`installing_from_source`.
 
@@ -91,3 +96,92 @@ due to an experimental dependency not being present.
    All Python decorators that skip tests will be defined in
    ``lib/iris/tests/__init__.py`` with a function name with a prefix of
    ``skip_``.
+
+
+.. _using nox:
+
+Using Nox for Testing Iris
+==========================
+
+Iris has adopted the use of the `nox`_ tool for automated testing on `cirrus-ci`_
+and also locally on the command-line for developers.
+
+`nox`_ is similar to `tox`_, but instead leverages the expressiveness and power of a Python
+configuration file rather than an `.ini` style file. As with `tox`_, `nox`_ can use `virtualenv`_
+to create isolated Python environments, but in addition also supports `conda`_ as a testing
+environment backend.
+
+
+Where is Nox Used?
+------------------
+
+Iris uses `nox`_ as a convenience to fully automate the process of executing the Iris tests, but also
+automates the process of:
+
+* building the documentation and executing the doc-tests
+* building the documentation gallery
+* running the documentation URL link check
+* linting the code-base
+* ensuring the code-base style conforms to the `black`_ standard
+
+
+You can perform all of these tasks manually yourself, however the onus is on you to first ensure
+that all of the required package dependencies are installed and available in the testing environment.
+
+`Nox`_ has been configured to automatically do this for you, and provides a means to easily replicate
+the remote testing behaviour of `cirrus-ci`_ locally for the developer.
+
+
+Installing Nox
+--------------
+
+We recommend installing `nox`_ using `conda`_. To install `nox`_ in a separate `conda`_ environment::
+
+  conda create -n nox -c conda-forge nox
+  conda activate nox
+
+To install `nox`_ in an existing active `conda`_ environment::
+
+  conda install -c conda-forge nox
+
+The `nox`_ package is also available on PyPI, however `nox`_ has been configured to use the `conda`_
+backend for Iris, so an installation of `conda`_ must always be available.
+
+
+Testing with Nox
+----------------
+
+The `nox`_ configuration file `noxfile.py` is available in the root ``iris`` project directory, and
+defines all the `nox`_ sessions (i.e., tasks) that may be performed. `nox`_ must always be executed
+from the ``iris`` root directory.
+
+To list the configured `nox`_ sessions for Iris::
+
+  nox --list
+
+To run the Iris tests for all configured versions of Python::
+
+  nox --session tests
+
+To build the Iris documentation specifically for Python 3.7::
+
+  nox --session doctest-3.7
+
+To run all the Iris `nox`_ sessions::
+
+  nox
+
+For further `nox`_ command-line options::
+
+  nox --help
+
+.. note:: `nox`_ will cache its testing environments in the `.nox` root ``iris`` project directory.
+
+
+.. _black: https://black.readthedocs.io/en/stable/
+.. _nox: https://nox.thea.codes/en/latest/
+.. _setuptools: https://setuptools.readthedocs.io/en/latest/
+.. _tox: https://tox.readthedocs.io/en/latest/
+.. _virtualenv: https://virtualenv.pypa.io/en/latest/
+.. _PyPI: https://pypi.org/project/nox/
+.. _v41.5.0: https://setuptools.readthedocs.io/en/latest/history.html#v41-5-0