Issue #24541: Drop test_inspect.test_eightteen unittest; update docs

Suggested by Martin Panter.

Doc/library/inspect.rst

@@ -28,7 +28,7 @@ Types and members
 -----------------
 
 The :func:`getmembers` function retrieves the members of an object such as a
-class or module. The sixteen functions whose names begin with "is" are mainly
+class or module. The functions whose names begin with "is" are mainly
 provided as convenient choices for the second argument to :func:`getmembers`.
 They also help you determine when you can expect to find the following special
 attributes:

Lib/test/test_inspect.py

@@ -91,14 +91,6 @@ def gen_coroutine_function_example(self):
 
 
 class TestPredicates(IsTestBase):
-    def test_eightteen(self):
-        count = len([x for x in dir(inspect) if x.startswith('is')])
-        # This test is here for remember you to update Doc/library/inspect.rst
-        # which claims there are 18 such functions
-        expected = 18
-        err_msg = "There are %d (not %d) is* functions" % (count, expected)
-        self.assertEqual(count, expected, err_msg)
-
 
     def test_excluding_predicates(self):
         global tb