fix TaskFactory and loop.create_task

[graingert]

No description provided.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mitmproxy (https://github.com/mitmproxy/mitmproxy)
- mitmproxy/utils/asyncio_utils.py:96: error: Unused "type: ignore" comment  [unused-ignore]

pyodide (https://github.com/pyodide/pyodide)
+ src/py/pyodide/webloop.py:425: error: Signature of "create_task" incompatible with supertype "AbstractEventLoop"  [override]
+ src/py/pyodide/webloop.py:425: note:      Superclass:
+ src/py/pyodide/webloop.py:425: note:          def [_T] create_task(self, coro: Coroutine[Any, Any, _T], *, name: str | None = ..., context: Context | None = ..., eager_start: bool | None = ...) -> Task[_T]
+ src/py/pyodide/webloop.py:425: note:      Subclass:
+ src/py/pyodide/webloop.py:425: note:          def [T] create_task(self, coro: Coroutine[T, Any, Any], *, name: str | None = ..., context: Context | None = ...) -> Task[T]

[srittau]

Considering that not all task factories have the eager_start argument (in particular the eager_task_factory), I think the best we can do is replace the eager_task_factory argument in the protocol with **kwargs: bool and explain in a comment that some factories support the eager_start argument and some don't.