Minimal howto run tests locally

doc/running-tests.rst

+Running the test suite
+----------------------
+
+Use the ``Makefile`` at the root of the repository.
+
+Prerequisite
+============
+
+On Linux, you should have ``libffi`` installed.
+
+For ubuntu, use::
+
+   sudo apt-get install -y libffi-dev
+
+Running tests
+=============
+
+By default, tests are run with Python 3.8::
+
+   make test
+
+
+To run tests with another version, use the ``PYTHON_VER`` environment
+variable::
+
+   PYTHON_VER=3.9 make test
+
+We still support the following versions.
+
+- 3.10
+- 3.9
+- 3.8
+- 3.7
+- 3.6
+- 3.5
+- 2.7
+- (3.11)