Warnings and errors not cleared on file closed

When one opens a python file that triggers errors and warning form linters, and the file is closed, the warnings and errors are not cleared from the Diagnostics panel until all python files are closed.

This behaviour pollutes the panel, specially when one opens a file from a third party library one can not modify.

Errors and warnings should be cleared for each closed file.

未指定文件或测试。打开一个包含 linter 警告或错误的 Python 文件，将其关闭，并观察 Diagnostics 面板，以重现该问题；跟踪文件关闭处理和 diagnostics 清理过程。完成标准是：每个已关闭文件的 diagnostics 都会被移除，而不要求关闭所有 Python 文件。

由索引模型根据 Issue 内容生成。