Optionally flag known mypyc compatibility issues in mypy

Feature

Mypyc is incompatible in some ways with CPython semantics (mypyc/mypyc#973 is a recent example). Add an optional error code to mypy that detects some of these incompatible features in code even when not using mypyc. We can also explain how to work around the issues in notes, or include links to documentation.

Here are some issues we could flag:

• Overriding methods in subclasses of built-in types such as int or str that mypyc can't use.
• Subclassing compiled classes that don't support interpreted subclasses. This would require knowledge of which modules will be compiled.
• Monkey patching native functions or methods. This is usually flagged already by mypy, but we could perhaps detect this in additional cases, add a note about mypyc when relevant, and use a separate error code.

Pitch

Code that isn't compiled with mypyc can trigger incompatible behavior or exceptions when interacting with mypyc-compiled modules, and this is confusing. These messages would make it easier to avoid using things that may cause issues with mypyc, if part of a project is compiled with mypyc.

まず mypyc#973 の例と、monkey patching に関する mypy の既存の診断を確認します。組み込み型のサブクラスでのオーバーライド、コンパイルされたクラスのサブクラス化、および関連する monkey patching を対象とするオプションのエラーコードの範囲を、必要な注記やドキュメントへのリンクも含めて定義します。選択した非互換性が mypyc を必要とせずに検出され、その動作がテストでカバーされれば完了です。