Round lut values where necessary

[radarhere]

Resolves #6187

The issue reports that float lut values raise an error in Python 3.10 for im.point().

This would be because _point calls getlist(),

Pillow/src/_imaging.c

Line 1428 in 4996f84

data = getlist(list, &n, wrong_number, TYPE_INT32);

which then calls PyLong_AsLong.

Pillow/src/_imaging.c

Lines 379 to 380 in 4996f84

	case TYPE_INT32:
	itemp = PyLong_AsLong(op);

https://docs.python.org/3/c-api/long.html#c.PyLong_AsLong

Changed in version 3.10: This function will no longer use __int__().

So this PR casts lut values to int first, except for the case where _point uses float values instead.