Consistently write in binary format in PPM tests

[radarhere]

test_file_ppm.py sometimes writes in binary format

Pillow/Tests/test_file_ppm.py

Lines 196 to 197 in be8a28d

	with open(path, "wb") as f:
	f.write(b"P3\n128 128\n255\n256")

and sometimes does not.

Pillow/Tests/test_file_ppm.py

Lines 243 to 244 in be8a28d

	with open(path, "w", encoding="utf-8") as f:
	f.write("P6")

This PR consistently writes in binary format in test_file_ppm.py.