Fixed #21235 -- Disabled savepoints for old versions of SQLite.

Thanks Ramiro for the report.

django/db/backends/sqlite3/base.py

@@ -393,12 +393,16 @@ class DatabaseWrapper(BaseDatabaseWrapper):
             BaseDatabaseWrapper.close(self)
 
     def _savepoint_allowed(self):
+        # Two conditions are required here:
+        # - A sufficiently recent version of SQLite to support savepoints,
+        # - Being in a transaction, which can only happen inside 'atomic'.
+
         # When 'isolation_level' is not None, sqlite3 commits before each
         # savepoint; it's a bug. When it is None, savepoints don't make sense
-        # because autocommit is enabled. The only exception is inside atomic
-        # blocks. To work around that bug, on SQLite, atomic starts a
+        # because autocommit is enabled. The only exception is inside 'atomic'
+        # blocks. To work around that bug, on SQLite, 'atomic' starts a
         # transaction explicitly rather than simply disable autocommit.
-        return self.in_atomic_block
+        return self.features.uses_savepoints and self.in_atomic_block
 
     def _set_autocommit(self, autocommit):
         if autocommit: