gh-154871: Fix asyncio.Task.get_context() crash

[BHUVANSH855]

Summary

Fix a crash in _asyncio.Task.get_context() when called on an uninitialized task.

Task instances created via __new__() (or subclasses that override __init__() without calling super().__init__()) leave task_context as NULL. Previously, get_context() unconditionally called Py_NewRef(self->task_context), which dereferenced a null pointer and crashed the interpreter.

This change checks whether task_context has been initialized before returning it. If it is NULL, get_context() now returns None, matching the behavior of the neighboring get_coro() and get_name() methods.

Changes

• Guard task_context before calling Py_NewRef().
• Return None when task_context is uninitialized.
• Add a regression test covering Task.__new__(Task).get_context() for the C implementation.
• Add a NEWS entry under the Library section.

Testing

• Built CPython successfully.
• Ran the new regression test:
  • ./python -m test test_asyncio.test_tasks -v -m test_get_context_uninitialized
• Ran the full asyncio task test module:
  • ./python -m test test_asyncio.test_tasks

All tests passed successfully.

Closes issue gh-154871

[johnslavik]

I'll change "Closes gh-154871" in your PR description to "Closes issue gh-154871", because the use of "closes" keyword would close the issue immediately after this PR is merged and before 3 other backport PRs of your change to our maintenance branches are merged. We want to close the issue after the backport PRs are merged.