Fix datetime.datetime not supporting __class_getitem__ in runtime context

[Copilot]

Plan: Add class_getitem support to datetime class

• Explore repository and understand the issue
• Identify that only datetime.datetime class needs class_getitem
• Add class_getitem to datetime.datetime (C implementation)
• Add class_getitem to datetime.datetime (pure Python implementation)
• Build and manually test the changes
• Create tests for the new functionality
• Run existing tests to ensure no breakage
• Run code review
• Run CodeQL security check
• Add documentation for the new feature
• Update examples and tests to use tzinfo instead of int

Summary

This PR adds __class_getitem__ support to the datetime.datetime class only, as requested. This enables the datetime class to be used with generic type syntax at runtime (e.g., datetime.datetime[tzinfo]).

The implementation:

• Adds __class_getitem__ to datetime.datetime in both C and pure Python implementations
• Follows the same pattern as other standard library types (Queue, etc.)
• Includes comprehensive test coverage using tzinfo as the generic parameter
• Does NOT add this functionality to other datetime module classes (date, time, timedelta, tzinfo, timezone)
• Includes documentation with a versionchanged note for Python 3.15, using tzinfo in the example

Original prompt

Please tackle using the generic datetime in a runtime context, since the actual runtime datetime object doesn't support class_getitem().

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[m-aciek]

@copilot Please add this only to datetime class, not to others in datetime module.

[Copilot]

@copilot Please add this only to datetime class, not to others in datetime module.

Done. I've updated the implementation to add __class_getitem__ only to the datetime.datetime class. The changes have been applied to both the C implementation and the pure Python implementation. All other datetime module classes (date, time, timedelta, tzinfo, timezone) no longer have this method.

Commit: aa78e5f

[m-aciek]

@copilot Is there any change in the documentation needed?

[Copilot]

@copilot Is there any change in the documentation needed?

Yes, I've added documentation for this feature. The datetime.rst file now includes a versionchanged directive for Python 3.15 that documents the new subscript operator support for creating generic aliases (e.g., datetime[int]). The note references PEP 585 and links to the Generic Alias Type section.

Commit: 3eeeb02

[m-aciek]

@copilot Please update an example and test with generic over tzinfo instead of int

[Copilot]

@copilot Please update an example and test with generic over tzinfo instead of int

Done. I've updated the documentation example to use datetime[tzinfo] instead of datetime[int], and updated the test to use tzinfo as the generic parameter in both single and multiple argument cases.

Commit: ffa9326