mypy default cache is not using XDG specification

mypy should follow the https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html specification for its default location of the cache directory, this means that its cache should be located somewhere under $XDG_CACHE_HOME or ~/.cache when environment variable is not defined.

The reasons for this is mostly about de-cluttering repositories, keeping cache files in a single location, making it easy to cleanup by the operating system.

Lots of tools adopted this specification even when have nothing to so with XDG itself, as it is portable and sets good practices.

まず、mypy のキャッシュディレクトリ設定を見つけ、現在のデフォルト値をリンク先の XDG Base Directory 仕様と比較します。XDG_CACHE_HOME が設定されている場合と設定されていない場合の動作を確認します。キャッシュがその変数の下、または変数が未設定の場合は ~/.cache の下に配置されれば完了です。