Add a comment about multi-version local testing

Adds a comment about how to run tests locally on multiple versions of python.

Ensures it is clear that it only works if particular version of python is installed on the users machine.

Recommends `pytest` as a possible way to manage multiple local versions of python.

Note the hyperlink out to `setup.cfg` is pinned to commit sha 1297c8c instead of pinning to main branch.

docs/text/how_to_contribute.rst

@@ -67,6 +67,18 @@ you have to:
     pytest
 
 
+To test changes across multiple versions of Python, run:
+
+
+.. code::
+
+    tox -r
+
+
+`tox -r` will execute tests for the Python versions specified in `setup.cfg <https://github.com/blue-yonder/tsfresh/blob/1297c8ca5bd6f8f23b4de50e3f052fb4ec1307f8/setup.cfg>`_ using the `envlist` variable. For example, if `envlist` is set to `py37, py38`, the test suite will run for Python 3.7 and 3.8 on the local development platform, assuming the binaries for those versions are available locally. The exact Python microversions (e.g. `3.7.1` vs `3.7.2`) depend on what is installed on the local development machine.
+
+A recommended way to manage multiple Python versions when testing locally is with `pyenv`, which enables organized installation and switching between versions.
+
 Documentation
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