Exclude .venv from Time Machine backups (#4599)

* Exclude .venv from Time Machine backups

* Remove leftover print statements

* Make backup exclusion test only run on darwin

* Add comment explaining backup exclusion xattr value

* Use plistlib to generate backup exclusion xattr value

* Only install xattr on darwin

* Add TODO comment

* Merge fix

* Only import xattr on macOS test

* Try fixing CI error

* Add type annotations

* Fix CI errors

* Add spacing pyproject.toml

Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>

* Move mypy ignore to `pyproject.toml`

Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>