bpo-23882: unittest: Drop PEP 420 support from discovery. (GH-29745)

Doc/library/unittest.rst

@@ -266,8 +266,7 @@ Test Discovery
 
 Unittest supports simple test discovery. In order to be compatible with test
 discovery, all of the test files must be :ref:`modules <tut-modules>` or
-:ref:`packages <tut-packages>` (including :term:`namespace packages
-<namespace package>`) importable from the top-level directory of
+:ref:`packages <tut-packages>` importable from the top-level directory of
 the project (this means that their filenames must be valid :ref:`identifiers
 <identifiers>`).
 
@@ -340,6 +339,24 @@ the `load_tests protocol`_.
    directory too (e.g.
    ``python -m unittest discover -s root/namespace -t root``).
 
+.. versionchanged:: 3.11
+   Python 3.11 dropped the :term:`namespace packages <namespace package>`
+   support. It has been broken since Python 3.7. Start directory and
+   subdirectories containing tests must be regular package that have
+   ``__init__.py`` file.
+
+   Directories containing start directory still can be a namespace package.
+   In this case, you need to specify start directory as dotted package name,
+   and target directory explicitly. For example::
+
+      # proj/  <-- current directory
+      #   namespace/
+      #     mypkg/
+      #       __init__.py
+      #       test_mypkg.py
+
+      python -m unittest discover -s namespace.mypkg -t .
+
 
 .. _organizing-tests:
 
@@ -1858,6 +1875,10 @@ Loading and running tests
          whether their path matches *pattern*, because it is impossible for
          a package name to match the default pattern.
 
+      .. versionchanged:: 3.11
+         *start_dir* can not be a :term:`namespace packages <namespace package>`.
+         It has been broken since Python 3.7 and Python 3.11 officially remove it.
+
 
    The following attributes of a :class:`TestLoader` can be configured either by
    subclassing or assignment on an instance: