[PR #8012/5f699bbb backport][3.10] Fix FileResponse doing blocking I/O in the event loop

[patchback]

This is a backport of PR #8012 as merged into master (5f699bb).

What do these changes do?

Fix FileResponse doing blocking I/O in the event loop

The call to os.stat was run in the executor but the call to is_file() was not. Additionally this resulted in two syscalls to stat since both is_file and stat will call stat under the hood. The code has been refactored to ensure there is only one stat syscall and it happens in the executor

Are there changes in behavior for the user?

No

Related issue number

discovered while investigating #8011 but not does fix the issue reported

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
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