Reflect it that `sys.lazy_modules` is no longer a flat set

[johnslavik]

Bug report

Bug description:

sys.lazy_modules is now a dict[str, set[str]], not a flat set. There's some places in the stdlib code that incorrectly suggests the older invariant.

Most notable example:

cpython/Lib/test/test_import/test_lazy_imports.py

Lines 457 to 459 in 70da972

	def test_lazy_modules_attribute_is_set(self):
	"""sys.lazy_modules should be a set per PEP 810."""
	self.assertIsInstance(sys.lazy_modules, dict)

The PEP needs updating as well -- https://peps.python.org/pep-0810/#lazy-import-mechanism -- @DinoV, I think this is something you were working on, or is it open for contrib?

I'm happy to pick this one up (i.e. fixing our code to reflect sys.lazy_modules is a dict) if you'd like that! (Please reassign otherwise.)

cc @pablogsal @DinoV @brittanyrey @Yhg1s

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response