gh-135386: Fix dbm.sqlite3 readonly open error by using immutable=1

@@ -69,6 +69,7 @@ def __init__(self, path, /, *, flag, mode):
 self._cx = sqlite3.connect(uri, autocommit=True, uri=True)
 except sqlite3.Error as exc:
 raise error(str(exc))
+
 self._readonly = (flag == "ro")
 # This is an optimization only; it's ok if it fails.
 if not self._readonly:

@@ -1,3 +1,4 @@
+import os
 import sys
 import unittest
 from contextlib import closing
@@ -89,6 +90,22 @@ def test_readonly_keys(self):
 def test_readonly_iter(self):
 self.assertEqual([k for k in self.db], [b"key1", b"key2"])
 
+def test_readonly_open_without_wal_shm(self):
+wal_path = self.filename + "-wal"
+shm_path = self.filename + "-shm"
+
+for suffix in wal_path, shm_path:
+try:
+os.remove(suffix)
+except FileNotFoundError:
+pass
+
+os.chmod(self.filename, 0o444)
+
+with dbm_sqlite3.open(self.filename, "r") as db:
+self.assertEqual(db[b"key1"], b"value1")
+self.assertEqual(db[b"key2"], b"value2")
+
 
 class ReadWrite(_SQLiteDbmTests):
 

@@ -0,0 +1 @@
+Fix :exc:`sqlite3.OperationalError` error when using :func:`dbm.open` with a read-only file object.