Fixed unclosed file warning in test_imageshow.py

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/11202062547/job/31137725975#step:12:4820

Tests/test_imageshow.py::test_ipythonviewer
/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/_pytest/python.py:159: ResourceWarning: unclosed file <_io.BufferedReader name='/Users/runner/work/Pillow/Pillow/Tests/images/hopper.ppm'>

The file is opened in hopper()

Pillow/Tests/helper.py

Lines 252 to 261 in 27c1bb2

	def hopper(mode: str | None = None) -> Image.Image:
	# Use caching to reduce reading from disk, but return a copy
	# so that the cached image isn't modified by the tests
	# (for fast, isolated, repeatable tests).
	if mode is None:
	# Always return fresh not-yet-loaded version of image.
	# Operations on not-yet-loaded images are a separate class of errors
	# that we should catch.
	return Image.open("Tests/images/hopper.ppm")

and then called in

Pillow/Tests/test_imageshow.py

Lines 120 to 121 in 27c1bb2

	im = hopper()
	assert test_viewer.show(im) == 1

Adding a context manager fixes the warning.