Remove some no-longer-needed compat code in test_assertion

(cherry picked from commit 20956b2)

Author: bluetech

testing/test_assertion.py

@@ -1,4 +1,4 @@
-import collections.abc as collections_abc
+import collections.abc
 import sys
 import textwrap
 from typing import Any
@@ -684,11 +684,8 @@ def test_frozenzet(self):
         assert len(expl) > 1
 
     def test_Sequence(self):
-        if not hasattr(collections_abc, "MutableSequence"):
-            pytest.skip("cannot import MutableSequence")
-        MutableSequence = collections_abc.MutableSequence
-
-        class TestSequence(MutableSequence):  # works with a Sequence subclass
+        # Test comparing with a Sequence subclass.
+        class TestSequence(collections.abc.MutableSequence):
             def __init__(self, iterable):
                 self.elements = list(iterable)