gh-94912: Added marker for non-standard coroutine function detection

[carltongibson]

This is ref the discussion on #94912 to add an official way of marking callables as coroutine functions where they would not otherwise be detected.

cc. @gvanrossum @andrewgodwin I've just had time this afternoon to block off some first tests, and docs. I wanted to break ground so we keep it moving. It's very consciously drafts: please make suggestions of how you think it should go.

@gvanrossum Can I ask for a pointer on how I'm meant to set co_flags for the _has_code_flag() check? 🤔

cpython/Lib/inspect.py

Lines 376 to 385 in c43714f

	def _has_code_flag(f, flag):
	"""Return true if ``f`` is a function (or a method or functools.partial
	wrapper wrapping a function) whose code object has the given ``flag``
	set in its flags."""
	while ismethod(f):
	f = f.__func__
	f = functools._unwrap_partial(f)
	if not (isfunction(f) or _signature_is_functionlike(f)):
	return False
	return bool(f.__code__.co_flags & flag)

Thanks!

First PR on CPython, so likely it's wrong 😊

• Clone and Build as per https://devguide.python.org.
• Run new just tests with % ./python.exe -m unittest test.test_inspect.TestPredicates
• Do we need to create a new ticket, or is make asyncio.iscoroutinefunction a deprecated alias of inspect.iscoroutinefunction and remove asyncio.coroutines._is_coroutine #94912 OK?

• Issue: make asyncio.iscoroutinefunction a deprecated alias of inspect.iscoroutinefunction and remove asyncio.coroutines._is_coroutine #94912