Explain how to switch to add native namespace

docs/reference-guide/native-namespaces.md

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+# Native namespace
+
+This document explains the steps needed to convert a python distribution from `pkg_resources` namespace to native namespaces.
+
+## Background
+
+Python, since Python 3.3, added support for native namespaces, see [PEP 420](https://peps.python.org/pep-0420/).
+
+Plone has been using `pkg_resources`-style namespaces, but they are deprecated in `setuptools`.
+
+`setuptools` is planning to remove `pkg_resources`'s namespaces support by end of 2025.
+
+[PLIP 3928](https://github.com/plone/Products.CMFPlone/issues/3928) is tracking the changes needed for Plone to adapt to the _new_ native namespaces.
+
+## Steps
+
+To convert a given (`$package`) python distribution to use native namespaces follow these steps.
+
+### Create maintenance branch
+
+!!! note
+    Only relevant for Plone core packages
+
+Clone the repository or ensure you are at latest changes:
+
+```shell
+git clone git@github.com:plone/$package
+cd $package
+# or update
+git fetch -p
+git checkout main # or master
+git rebase
+```
+
+Find out what's the last release and create a branch for it:
+
+```shell
+# list all tags
+git for-each-ref --sort=taggerdate --format '%(tag)'
+# get the last tag's major number
+MAJOR=`git for-each-ref --sort=taggerdate --format '%(tag)' | tail -n1 | cut -d"." -f1`
+# create a branch for it
+git checkout -b $MAJOR.x
+# push the newly created branch
+git push $MAJOR.x
+```
+
+### Update buildout.coredev
+
+!!! note
+    Only relevant for Plone core packages
+
+Update `buildout.coredev`'s branch 6.1 to use the newly created branch
+
+```shell
+# go to the repository or clone it
+cd buildout.coredev
+# ensure you are at latest changes
+git fetch -p
+git checkout 6.1
+git rebase
+# update the branch being used by the python distribution
+sed -i "s/$package.git branch=master/$package.git branch=$MAJOR.x/" sources.cfg
+# add the changes, commit and push
+git add sources.cfg
+git commit -m"chore: use branch $MAJOR.x for $package"
+git push
+```
+
+!!! note
+    To lower the amount of builds in Jenkins, either do a few at a time or add a `[ci-skip]` on the commit message
+
+### Numbers before
+
+One risk of changing to the native namespaces is that some files, or tests, might not be left behind.
+
+To ensure all tests and files are kept after the switch, gather the numbers before the change:
+
+Get the list of tests:
+
+```shell
+tox run -e test -- --list-tests
+```
+
+!!! note
+    Adapt to whichever way you are using to run the tests.
+    The above is meant for repositories that follow `plone.meta` conventions.
+    The `--list-tests` comes from `zope.testrunner`, if you are using that but not `plone.meta` you can use that as well.
+
+Create a distribution to get a listing of how many files are currently packaged:
+
+```shell
+uvx --from build pyproject-build
+```
+
+A `dist` folder is created with two archives in it: a `.tar.gz` and a `.whl`.
+
+To get the number of files on them run the following commands:
+
+```shell
+python -c "import tarfile; print(len(tarfile.open('dist/$package.tar.gz', 'r:gz').getnames()))"
+python -c "from zipfile import ZipFile; print(len(ZipFile('dist/$package.whl').namelist()))"
+```
+
+Keep these numbers around for later.
+
+### Build backend
+
+To ensure the python continues to build, ensure that `setuptools` is defined as its build backend.
+
+For that inspect the `pyproject.toml`, it should have these lines:
+
+```toml
+[build-system]
+requires = ["setuptools>=68.2,<80", "wheel"]
+```
+
+If they are not there, add them, commit and push the changes.
+
+### Convert to native namespace
+
+Use `plone.meta`'s `switch-to-pep420` script:
+
+```shell
+cd $package
+uvx --from plone.meta switch-to-pep420 .
+```
+
+### Update the test matrix
+
+!!! note
+    Only relevant for Plone core packages
+
+As the switch to the native namespace has to be coordinated, all python distributions need to be only for the same Plone version, in this case it was decided to do it for the Plone 6.2 version.
+
+Thus, we need to ensure that the test matrix, only tests it against this Plone version.
+
+For that, update `.meta.toml` with the following changes:
+
+```toml
+[tox]
+test_matrix = {"6.2" = ["*"]}
+```
+
+And update the scaffolding files with `plone.meta`:
+
+```shell
+uvx --from plone.meta config-package .
+```
+
+Review the changes and ensure all changes are sound.
+
+!!! note
+    If the diff is quite big, run `config-package` before all the changes, get that on a Pull Request, approved and merged and then do a follow up Pull Request to move to native namespace.
+
+### Compare numbers
+
+Get the list of tests, like before and compare the lists to ensure that the same amount of tests are found.
+
+```shell
+tox run -e test -- --list-tests
+```
+
+Likewise, create the distribution files and compare the numbers with the previous run:
+
+```shell
+python -c "import tarfile; print(len(tarfile.open('dist/$package.tar.gz', 'r:gz').getnames()))"
+python -c "from zipfile import ZipFile; print(len(ZipFile('dist/$package.whl').namelist()))"
+```
+
+If all numbers match, review the changes once more, push the branch with the changes for others to review it.