Allow instantiating type[None] / types.NoneType

[mmaxjr]

Summary

analyze_type_type_callee() in checkexpr.py had no branch for mypy's internal NoneType. As a result, calling a callee of type type[None] incorrectly produced Cannot instantiate type "type[None]", even though this is valid at runtime (type(None)() and NoneType() both return None).

Repro from the issue:

from types import NoneType

type(None)()
NoneType()

def f(n: type[None]):
    n()

def g(n: type[NoneType]):
    n()

f(NoneType)
g(NoneType)

Before this change, mypy reported 4 spurious Cannot instantiate type "type[None]" errors (plus the expected NoneType should not be used as a type note). After this change, only the expected note remains.

Fix: add a case in analyze_type_type_callee that, for NoneType items, returns a nullary CallableType with return type NoneType(), mirroring how type(None) behaves at runtime.

Test plan

• Added testTypeCanInstantiateNoneType to test-data/unit/check-classes.test
• pytest mypy/test/testcheck.py -k "check-classes or check-python310" -> 810 passed
• pytest mypy/test/testcheck.py (full suite) -> 8175 passed
• mypy --config-file mypy_self_check.ini mypy/checkexpr.py -> no issues
• pre-commit run on changed files -> passed

Fixes #19660

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅