Improve `test_pickling` in `test_ast`

[sobolevn]

Here's how it is defined right now:

cpython/Lib/test/test_ast.py

Lines 640 to 655 in 837ba05

	def test_pickling(self):
	import pickle
	mods = [pickle]
	try:
	import cPickle
	mods.append(cPickle)
	except ImportError:
	pass
	protocols = [0, 1, 2]
	for mod in mods:
	for protocol in protocols:
	for ast in (compile(i, "?", "exec", 0x400) for i in exec_tests):
	ast2 = mod.loads(mod.dumps(ast, protocol))
	self.assertEqual(to_tuple(ast2), to_tuple(ast))
	def test_invalid_sum(self):

I see two major problems here:

1. cPickle does not exist anymore (since 3.0), so it always fails with ImportError. If we want to be sure that both Python and C versions behave the same - we can surely do that with import_helper.import_fresh_module or we can drop this part and only keep pickle module direct usage (as other modules do). I prefer the second option, because of how other things are tested. But, in the end it is up to maintainers to decide :)
2. Not all pickle protocols are tested: we need to use pickle.HIGHEST_PROTOCOL instead of just three items

PR is incoming!

Linked PRs

• gh-100882: Improve test_pickling case in test_ast.py #100883

[sobolevn]

Solved by #100883