Fixed #27600 -- Suppressed the REPL during shell's reading from stdin.

Thanks Adam Chainz for review and guidance.

tests/shell/tests.py

@@ -1,7 +1,10 @@
+import sys
+import unittest
+
 from django import __version__
 from django.core.management import call_command
-from django.test import SimpleTestCase
-from django.test.utils import patch_logger
+from django.test import SimpleTestCase, mock
+from django.test.utils import captured_stdin, captured_stdout, patch_logger
 
 
 class ShellCommandTestCase(SimpleTestCase):
@@ -17,3 +20,12 @@ class ShellCommandTestCase(SimpleTestCase):
             )
             self.assertEqual(len(logger), 1)
             self.assertEqual(logger[0], __version__)
+
+    @unittest.skipIf(sys.platform == 'win32', "Windows select() doesn't support file descriptors.")
+    @mock.patch('django.core.management.commands.shell.select')
+    def test_stdin_read(self, select):
+        with captured_stdin() as stdin, captured_stdout() as stdout:
+            stdin.write('print(100)\n')
+            stdin.seek(0)
+            call_command('shell')
+        self.assertEqual(stdout.getvalue().strip(), '100')