[stdlib] Allow loop.create_task eager_start on Python 3.13

[nightcityblade]

Fixes #15269.

Python 3.13.3 added arbitrary keyword forwarding to event-loop create_task, which allows eager_start to be passed to a compatible task factory. Move the existing loop-specific eager_start signature gate from Python 3.14 to Python 3.13 while leaving the high-level asyncio.create_task and TaskGroup.create_task gates unchanged.

Add regression coverage for both AbstractEventLoop and BaseEventLoop.

Tests:

• pre-commit run --files stdlib/asyncio/base_events.pyi stdlib/asyncio/events.pyi stdlib/@tests/test_cases/asyncio/check_task.py
• python tests/mypy_test.py stdlib/asyncio -p 3.13
• python tests/pyright_test.py stdlib/asyncio --pythonversion 3.13 -p pyrightconfig.stricter.json
• python tests/pyright_test.py stdlib/@tests/test_cases/asyncio/check_task.py --pythonversion 3.13 -p pyrightconfig.testcases.json
• python tests/regr_test.py stdlib -p 3.13

[nightcityblade]

Refreshed the branch with current main in 6e276e8, which brings in the upstream fixes for both unrelated failures from the original run: the Python 3.15 _ctypes.dllist stub and the stub_uploader dependency pin. After the merge, pre-commit and structure checks passed; mypy on stdlib/asyncio passed for Python 3.13 and 3.14; the stdlib regression cases passed for 3.13 and 3.14; ty passed for stdlib/asyncio on 3.13; and the Pyright regression suite passed on 3.13. The host-dependent stdlib stubtest is not part of that proof because this local macOS Python lacks Tk and several curses runtime symbols; current main CI is green and the refreshed PR CI will provide the full matrix.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉