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.

No file or test is named. Reproduce the issue by opening a Python file with linter warnings or errors, closing it, and observing the Diagnostics panel; trace the file-close handling and diagnostics cleanup. Done means diagnostics for each closed file are removed without requiring all Python files to close.

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