gh-113093: Add mode parameter to shelve.open

[algonix01w-ai]

Description

This PR adds the missing mode parameter to shelve.open() and the DbfilenameShelf class.

Currently, shelve.open does not allow users to specify file permissions, even though the underlying dbm.open supports a mode parameter. This change ensures consistency between the shelve and dbm modules, allowing users to set custom file permissions (e.g., restricted access) when creating a shelf database.

Changes

• Lib/shelve.py:
  • Updated DbfilenameShelf.__init__ to accept mode as a keyword-only argument.
  • Updated shelve.open to accept mode and pass it to DbfilenameShelf.
• Lib/test/test_shelve.py:
  • Added test_mode using tempfile.TemporaryDirectory to verify that the mode parameter is accepted and correctly passed through without errors.
• Doc/library/shelve.rst:
  • Updated documentation signature for open() and added a versionchanged notice for 3.15.

Linked Issue

Fixes gh-113093

Tests

Ran python.bat -m test test_shelve on Windows 11. All tests passed successfully.

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[da-woods]

There looks to be another recent PR on this #148662