Fixed #23433 -- Deprecated django-admin.py entry point in favor of django-admin.

Unify on the entry point created by setuptools entry_points feature.

tests/admin_scripts/test_django_admin_py.py

@@ -0,0 +1,37 @@
+import subprocess
+import sys
+from pathlib import Path
+
+import django
+from django.test import SimpleTestCase
+
+
+class DeprecationTests(SimpleTestCase):
+    DEPRECATION_MESSAGE = (
+        b'RemovedInDjango40Warning: django-admin.py is deprecated in favor of '
+        b'django-admin.'
+    )
+
+    def _run_test(self, args):
+        p = subprocess.run(
+            [sys.executable, *args],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            check=True,
+        )
+        return p.stdout, p.stderr
+
+    def test_django_admin_py_deprecated(self):
+        django_admin_py = Path(django.__file__).parent / 'bin' / 'django-admin.py'
+        _, err = self._run_test(['-Wd', django_admin_py, '--version'])
+        self.assertIn(self.DEPRECATION_MESSAGE, err)
+
+    def test_main_not_deprecated(self):
+        _, err = self._run_test(['-Wd', '-m', 'django', '--version'])
+        self.assertNotIn(self.DEPRECATION_MESSAGE, err)
+
+    def test_django_admin_py_equivalent_main(self):
+        django_admin_py = Path(django.__file__).parent / 'bin' / 'django-admin.py'
+        django_admin_py_out, _ = self._run_test([django_admin_py, '--version'])
+        django_out, _ = self._run_test(['-m', 'django', '--version'])
+        self.assertEqual(django_admin_py_out, django_out)