Improve `test_inspect`

While working on #103406 I've noticed that there are some problems with test_inspect that we can easily fix.

List of problems I found:

1. 	f3 = self.makeCallable('**c')
   	self.assertEqualException(f3, '1, 2')
   	self.assertEqualException(f3, '1, 2, a=1, b=2')
   	f4 = self.makeCallable('*, a, b=0')
   	self.assertEqualException(f3, '1, 2')
   	self.assertEqualException(f3, '1, 2, a=1, b=2')

   Here f4 is not ever used, but f3 has duplicate asserts. It looks like a copy-paste error to me. I propose to add asserts for f4 as well

2. 	# XXX: Python inconsistency
   	# - for functions and bound methods: unexpected keyword 'c'
   	# - for unbound methods: multiple values for keyword 'a'
   	#self.assertEqualException(f, '1, c=3, a=2')

   Unbound methods do not exist anymore, so this comment is outdated. Probably, it should be removed and assert should be restored

3. 	# XXX: success of this one depends on dict order
   	## self.assertEqualException(f, '2, 3, 4, a=1, c=5')

   It says that the result depends on the dict order. But, dicts are ordered now. Let's see if that possible to uncomment / modernize this test somehow

4. psig = inspect.signature(partial(partial(test, 1), 2))

   This variable is not needed, because it is not used and there's an identical test right above it:

   	self.assertEqual(self.signature(partial(partial(test, 1), 2)),
   	((('c', ..., int, "positional_or_keyword"),),
   	42))

5. There are also several unsused variables that can be removed durin this cleanup

Here's my PR :)

Linked PRs

• gh-103553: Improve test_inspect: add more assertions, remove unused #103554
• [3.11] gh-103553: Improve test_inspect: add more assertions, remove unused (GH-103554) #103568