Explain that the contributors should base changes against main (#983)

This isn't a particularly common issue but every now and then somebody will
waste their time by preparing a change against a maintenance branch instead of
working with `main` from the get-go.

This change serves as a reminder to contributors to think about the `main`
branch first.

Author: ambv

getting-started/pull-request-lifecycle.rst

@@ -29,7 +29,8 @@ Here is a quick overview of how you can contribute to CPython:
 
 #. `Create an issue`_ that describes your change [*]_
 
-#. :ref:`Create a new branch in Git <pullrequest-steps>`
+#. :ref:`Create a new branch in Git <pullrequest-steps>` from the
+   ``main`` branch
 
 #. Work on changes (e.g. fix a bug or add a new feature)
 
@@ -79,10 +80,16 @@ You should have already :ref:`set up your system <setup>`,
 
      git fetch upstream
 
-* Create a new branch in your local clone::
+* Create a new branch in your local clone from the ``main`` branch
+  in the upstream repository::
 
      git checkout -b <branch-name> upstream/main
 
+  .. note::
+     Almost all changes to Python, including bug fixes, should first be
+     made against the ``main`` branch. See :ref:`here <good-prs>` for
+     more details.
+
 * Make changes to the code, and use ``git status`` and ``git diff`` to see them.
 
   (Learn more about :ref:`good-prs`)
@@ -180,40 +187,50 @@ Making Good PRs
 When creating a pull request for submission, there are several things that you
 should do to help ensure that your pull request is accepted.
 
-First, make sure to follow Python's style guidelines. For Python code you
-should follow :PEP:`8`, and for C code you should follow :PEP:`7`. If you have
-one or two discrepancies those can be fixed by the core developer who merges
-your pull request. But if you have systematic deviations from the style guides
-your pull request will be put on hold until you fix the formatting issues.
-
-.. note::
-   Pull requests with only code formatting changes are usually rejected. On the other
-   hand, fixes for typos and grammar errors in documents and docstrings are
-   welcome.
-
-Second, be aware of backwards-compatibility considerations. While the core
-developer who eventually handles your pull request will make the final call on
-whether something is acceptable, thinking about backwards-compatibility early
-will help prevent having your pull request rejected on these grounds. Put
-yourself in the shoes of someone whose code will be broken by the change(s)
-introduced by the pull request. It is quite likely that any change made will
-break someone's code, so you need to have a good reason to make a change as
-you will be forcing someone to update their code. (This obviously does not
-apply to new classes or functions; new arguments should be optional and have
-default values which maintain the existing behavior.) If in doubt, have a look
-at :PEP:`387` or :ref:`discuss <communication>` the issue with experienced
-developers.
-
-Third, make sure you have proper tests to verify your pull request works as
-expected. Pull requests will not be accepted without the proper tests!
-
-Fourth, make sure the entire test suite :ref:`runs <runtests>` **without
-failure** because of your changes.  It is not sufficient to only run whichever
-test seems impacted by your changes, because there might be interferences
-unknown to you between your changes and some other part of the interpreter.
-
-Fifth, proper :ref:`documentation <documenting>`
-additions/changes should be included.
+#. **Make your change against the right version of Python.** In general all
+   changes are made against the ``main`` branch first. This includes bug fixes.
+   After the change is merged there, it will be :ref:`ported back <branch-merge>`
+   to older :ref:`maintenance releases <branchstatus>` as well. That way we
+   ensure all affected versions are handled. Therefore, basing a new change
+   directly on a maintenance branch is only used in specific circumstances,
+   for instance when that change does not apply to ``main`` or the change
+   requires a different approach in an older Python version compared to
+   ``main``.
+
+#. **Make sure to follow Python's style guidelines.** For Python code you
+   should follow :PEP:`8`, and for C code you should follow :PEP:`7`. If you have
+   one or two discrepancies those can be fixed by the core developer who merges
+   your pull request. But if you have systematic deviations from the style guides
+   your pull request will be put on hold until you fix the formatting issues.
+
+   .. note::
+      Pull requests with only code formatting changes are usually rejected. On
+      the other hand, fixes for typos and grammar errors in documents and
+      docstrings are welcome.
+
+#. **Be aware of backwards-compatibility considerations.** While the core
+   developer who eventually handles your pull request will make the final call on
+   whether something is acceptable, thinking about backwards-compatibility early
+   will help prevent having your pull request rejected on these grounds. Put
+   yourself in the shoes of someone whose code will be broken by the change(s)
+   introduced by the pull request. It is quite likely that any change made will
+   break someone's code, so you need to have a good reason to make a change as
+   you will be forcing someone to update their code. (This obviously does not
+   apply to new classes or functions; new arguments should be optional and have
+   default values which maintain the existing behavior.) If in doubt, have a look
+   at :PEP:`387` or :ref:`discuss <communication>` the issue with experienced
+   developers.
+
+#. **Make sure you have proper tests** to verify your pull request works as
+   expected. Pull requests will not be accepted without the proper tests!
+
+#. **Make sure all tests pass.** The entire test suite needs to
+   :ref:`run <runtests>` **without failure** because of your changes.
+   It is not sufficient to only run whichever test seems impacted by your
+   changes, because there might be interferences unknown to you between your
+   changes and some other part of the interpreter.
+
+#. Proper :ref:`documentation <documenting>` additions/changes should be included.
 
 
 .. _patchcheck: