There are no tests for mypy.plugins.proper_plugin

There don't seem to be any tests for mypy.plugins.proper_plugin, which was manually tested for its addition in https://github.com/python/mypy/pull/16036.

Since mypy apparently owns this code, we should test it.

I've figured out this works as a unit test except that we can't import mypy.types in a unit test.

[case properPlugin]
# flags: --config=tmp/foo.ini
from mypy.types import Type, CallableType
result: Type | None = None
isinstance(result, CallableType)                      
[file foo.ini]
\[mypy]
plugins=mypy.plugins.proper_plugin
[builtins fixtures/isinstancelist.pyi]

I've recently realized this plugin parents its notes wrong, so I'd like to include a regression test for that, but I am having a hard time figuring out how to trigger it.

Beginne mit mypy.plugins.proper_plugin und dem bereitgestellten Unit-Test-Fall und untersuche dann, wie sich das Verhalten der falsch zugeordneten Notizen auslösen lässt. Bestätige, wie sich die Import-Einschränkung von mypy.types auf den Testaufbau auswirkt, und betrachte die Arbeit als abgeschlossen, sobald die Tests das Plugin und das Regressionsszenario abdecken.