apertis-pkg-merge-updates: don't use Path objects as a context manager

This is deprecated and will be removed in python 3.13.

/usr/bin/apertis-pkg-merge-updates:38: DeprecationWarning: pathlib.Path.__enter__() is deprecated and scheduled for removal in Python 3.13; Path objects as a context manager is a no-op
/usr/bin/apertis-pkg-merge-updates:75: DeprecationWarning: pathlib.Path.__enter__() is deprecated and scheduled for removal in Python 3.13; Path objects as a context manager is a no-op

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>