MYPYPATH should override mypy_path in config file (not extend it)

Feature

The MYPYPATH environment variable should override the mypy_path config file option (not extend it).

Pitch

I propose the ability to override the mypy_path config file option with the the MYPYPATH environment variable. To make Visual Studio Code work with the Mypy extension for namespace packages, it is necessary to set mypy_path and explicit_package_bases, whereas I would like to disable the mypy_path option when running Tox (to typecheck the installed package). It is usually the case that environment variables have higher precedence over config file options, and this change would make Mypy consistent with the expected behavior.

まず、mypy の設定解析と、MYPYPATH および mypy_path の既存の処理を見つけ、次に設定の優先順位または環境変数を対象とするテストを探します。変更する前に現在の動作を確認してください。MYPYPATH が設定ファイルの mypy_path の値を置き換え、既存の動作と関連するオプションが引き続きテストでカバーされていれば完了です。