Clarify expectations on keeping Python versions

According to the PTI it was not clear whether Python versions should
be kept whenever new platform is being adopted.

This change adds recommendation for libraries and other projects
to keep older platforms, including Python.

Change-Id: Ie2bb6c96e39eb21ab42132774699f254c7f476a9

reference/project-testing-interface.rst

@@ -32,6 +32,14 @@ project:
 Tools and approaches vary by language, please choose which language is
 relevant to you.
 
+.. note::
+
+  Project Testing Interface defines a *minimal* list of platforms that
+  projects should test against. Having a more extensive set of tested platforms
+  (for instance, keeping older platforms that are not yet EOL) is both allowed
+  and encouraged.
+
+
 .. _pti-documentation:
 
 Documentation

reference/pti/python.rst

@@ -26,11 +26,19 @@ tree:
 - ``sphinx-build -W -b html doc/source doc/build``
 
 The Python 3 version may change from cycle to cycle. Projects should
-target the following, replacing `3x` with the :ref:`tested Python 3 runtimes
-<pti-tested-runtimes>` for the current development cycle:
+target the following, extending supported Python `3x` with the
+:ref:`tested Python 3 runtimes <pti-tested-runtimes>` for the current
+development cycle:
 
 - ``tox -e py3x``
 
+Projects should avoid removing Python versions that have not reached
+`End Of Life <https://devguide.python.org/versions/>`_ without a solid
+reason. It is recommended to keep compatability with older Python versions
+as long as possible.
+While CI coverage of Python versions that are not mentioned in PTI can be reduced,
+such reduction is not mandatory.
+
 Projects that are translated should also support:
 
 - ``tox -e venv python setup.py extract_messages``