ENH: switch mpl_toolkits to implicit namespace package (PEP 420)

Author: neutrinoceros

doc/users/next_whats_new/mpl_toolkit_pep420.rst

@@ -0,0 +1,6 @@
+``matplotlib.mpl_toolkits`` is now an implicit namespace package
+----------------------------------------------------------------
+
+Following the deprecation of ``pkg_resources.declare_namespace`` in ``setuptools`` 67.3.0,
+``matplotlib.mpl_toolkits`` is now implemented as an implicit namespace, following
+`PEP 420 <https://peps.python.org/pep-0420/>`_.

lib/mpl_toolkits/__init__.py

@@ -29,7 +29,7 @@
 import shutil
 import subprocess
 
-from setuptools import setup, find_packages, Distribution, Extension
+from setuptools import setup, find_namespace_packages, Distribution, Extension
 import setuptools.command.build_ext
 import setuptools.command.build_py
 import setuptools.command.sdist
@@ -301,8 +301,10 @@ def make_release_tree(self, base_dir, files):
     ],
 
     package_dir={"": "lib"},
-    packages=find_packages("lib"),
-    namespace_packages=["mpl_toolkits"],
+    packages=find_namespace_packages(
+        where="lib",
+        exclude=["*baseline_images*", "*tinypages*", "*mpl-data*", "*web_backend*"],
+    ),
     py_modules=["pylab"],
     # Dummy extension to trigger build_ext, which will swap it out with
     # real extensions that can depend on numpy for the build.