Improve `pickle` protocols coverage in `test_slice`

[sobolevn]

While working on #100817 I've noticed that slice is only tested with 3 pickle protocols. See

cpython/Lib/test/test_slice.py

Lines 237 to 243 in 8dd2766

	def test_pickle(self):
	s = slice(10, 20, 3)
	for protocol in (0,1,2):
	t = loads(dumps(s, protocol))
	self.assertEqual(s, t)
	self.assertEqual(s.indices(15), t.indices(15))
	self.assertNotEqual(id(s), id(t))

I don't think we need to limited supported protocols. Let's test all protocols!

Linked PRs

• gh-100931: Test all pickle protocols in test_slice #100932
• [3.10] gh-100931: Test all pickle protocols in test_slice (GH-100932). #100978
• [3.11] gh-100931: Test all pickle protocols in test_slice (GH-100932). #100979

[AlexWaygood]

Thanks!