Update py27/py34 deprecation docs

I've updated the text and incorporated the topics from #5275, so this
can now be part of the official docs, and #5275 can be closed/unpinned.

Closes #5275

doc/en/py27-py34-deprecation.rst

@@ -1,31 +1,97 @@
-Python 2.7 and 3.4 support plan
+Python 2.7 and 3.4 support
-===============================
+==========================
 
-Python 2.7 EOL is fast approaching, with
+It is demanding on the maintainers of an open source project to support many Python versions, as
-upstream support `ending in 2020 <https://legacy.python.org/dev/peps/pep-0373/#id4>`__.
+there's extra cost of keeping code compatible between all versions, while holding back on
-Python 3.4's last release is scheduled for
+features only made possible on newer Python versions.
-`March 2019 <https://www.python.org/dev/peps/pep-0429/#release-schedule>`__. pytest is one of
-the participating projects of the https://python3statement.org.
 
-The **pytest 4.6** series is the last to support Python 2.7 and 3.4, and was released in
+In case of Python 2 and 3, the difference between the languages makes it even more prominent,
-**June 2019**. **pytest 5.0** and onwards will support only Python 3.5+.
+because many new Python 3 features cannot be used in a Python 2/3 compatible code base.
 
-Thanks to the `python_requires`_ ``setuptools`` option,
+Python 2.7 EOL has been reached `in 2020 <https://legacy.python.org/dev/peps/pep-0373/#id4>`__, with
-Python 2.7 and Python 3.4 users using a modern ``pip`` version
+the last release planned for mid-April, 2020.
-will install the last pytest ``4.6`` version automatically even if ``5.0`` or later
+
+Python 3.4 EOL has been reached `in 2019 <https://www.python.org/dev/peps/pep-0429/#release-schedule>`__, with the last release made in March, 2019.
+
+For those reasons, in Jun 2019 it was decided that **pytest 4.6** series will be the last to support Python 2.7 and 3.4.
+
+What this means for general users
+---------------------------------
+
+Thanks to the `python_requires`_ setuptools option,
+Python 2.7 and Python 3.4 users using a modern pip version
+will install the last pytest 4.6.X version automatically even if 5.0 or later versions
 are available on PyPI.
 
-While pytest ``5.0`` will be the new mainstream and development version, until **January 2020**
+Users should ensure they are using the latest pip and setuptools versions for this to work.
-the pytest core team plans to make bug-fix releases of the pytest ``4.6`` series by
-back-porting patches to the ``4.6-maintenance`` branch that affect Python 2 users.
 
-**After 2020**, the core team will no longer actively backport patches, but the ``4.6-maintenance``
+Maintenance of 4.6.X versions
-branch will continue to exist so the community itself can contribute patches. The core team will
+-----------------------------
-be happy to accept those patches and make new ``4.6`` releases **until mid-2020**.
+
+Until January 2020, the pytest core team ported many bug-fixes from the main release into the
+``4.6-maintenance`` branch, with several 4.6.X releases being made along the year.
+
+From now on, the core team will **no longer actively backport patches**, but the ``4.6-maintenance``
+branch will continue to exist so the community itself can contribute patches.
+
+The core team will be happy to accept those patches, and make new 4.6.X releases **until mid-2020**
+(but consider that date as a ballpark, after that date the team might still decide to make new releases
+for critical bugs).
 
 .. _`python_requires`: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires
 
-Technical Aspects
+Technical aspects
------------------
+~~~~~~~~~~~~~~~~~
 
-The technical aspects of the Python 2.7 and 3.4 support plan (such as when releases will occur, how to backport fixes, etc) is described in issue `#5275 <https://github.com/pytest-dev/pytest/issues/5275>`__.
+(This section is a transcript from `#5275 <https://github.com/pytest-dev/pytest/issues/5275>`__).
+
+In this section we describe the technical aspects of the Python 2.7 and 3.4 support plan.
+
+What goes into 4.6.X releases
++++++++++++++++++++++++++++++
+
+New 4.6.X releases will contain bug fixes only.
+
+When will 4.6.X releases happen
++++++++++++++++++++++++++++++++
+
+New 4.6.X releases will happen after we have a few bugs in place to release, or if a few weeks have
+passed (say a single bug has been fixed a month after the latest 4.6.X release).
+
+No hard rules here, just ballpark.
+
+Who will handle applying bug fixes
+++++++++++++++++++++++++++++++++++
+
+We core maintainers expect that people still using Python 2.7/3.4 and being affected by
+bugs to step up and provide patches and/or port bug fixes from the active branches.
+
+We will be happy to guide users interested in doing so, so please don't hesitate to ask.
+
+**Backporting changes into 4.6**
+
+Please follow these instructions:
+
+#. ``git fetch --all --prune``
+
+#. ``git checkout origin/4.6-maintenance -b backport-XXXX`` # use the PR number here
+
+#. Locate the merge commit on the PR, in the *merged* message, for example:
+
+    nicoddemus merged commit 0f8b462 into pytest-dev:features
+
+#. ``git cherry-pick -m1 REVISION`` # use the revision you found above (``0f8b462``).
+
+#. Open a PR targeting ``4.6-maintenance``:
+
+   * Prefix the message with ``[4.6]`` so it is an obvious backport
+   * Delete the PR body, it usually contains a duplicate commit message.
+
+**Providing new PRs to 4.6**
+
+Fresh pull requests to ``4.6-maintenance`` will be accepted provided that
+the equivalent code in the active branches does not contain that bug (for example, a bug is specific
+to Python 2 only).
+
+Bug fixes that also happen in the mainstream version should be first fixed
+there, and then backported as per instructions above.