Add support for Python 3.15

[radarhere]

Pillow/setup.py

Lines 60 to 71 in 3ce6812

	if sys.platform == "win32" and sys.version_info >= (3, 15):
	import atexit
	atexit.register(
	lambda: warnings.warn(
	f"Pillow {PILLOW_VERSION} does not support Python "
	f"{sys.version_info.major}.{sys.version_info.minor} and does not provide "
	"prebuilt Windows binaries. We do not recommend building from source on "
	"Windows.",
	RuntimeWarning,
	)
	)

Since Python 3.15 has now reached beta, we would like users to be able to install and use it, so let's increment the version check.

[hugovk]

Thanks!