Remove backports of unittest matchers

This is unnecessary since we only support Python 3.

tests/common.py

@@ -3,7 +3,6 @@
 import errno
 import functools
 import os
-import sys
 import types
 
 from av.datasets import fate as fate_suite
@@ -159,42 +158,3 @@ def assertImagesAlmostEqual(self, a, b, epsilon=0.1, *args):
                 self.fail(
                     "images differed by %s at index %d; %s %s" % (diff, i, ax, bx)
                 )
-
-    # Add some of the unittest methods that we love from 2.7.
-    if sys.version_info < (2, 7):
-
-        def assertIs(self, a, b, msg=None):
-            if a is not b:
-                self.fail(
-                    msg
-                    or "%r at 0x%x is not %r at 0x%x; %r is not %r"
-                    % (type(a), id(a), type(b), id(b), a, b)
-                )
-
-        def assertIsNot(self, a, b, msg=None):
-            if a is b:
-                self.fail(msg or "both are %r at 0x%x; %r" % (type(a), id(a), a))
-
-        def assertIsNone(self, x, msg=None):
-            if x is not None:
-                self.fail(msg or "is not None; %r" % x)
-
-        def assertIsNotNone(self, x, msg=None):
-            if x is None:
-                self.fail(msg or "is None; %r" % x)
-
-        def assertIn(self, a, b, msg=None):
-            if a not in b:
-                self.fail(msg or "%r not in %r" % (a, b))
-
-        def assertNotIn(self, a, b, msg=None):
-            if a in b:
-                self.fail(msg or "%r in %r" % (a, b))
-
-        def assertIsInstance(self, instance, types, msg=None):
-            if not isinstance(instance, types):
-                self.fail(msg or "not an instance of %r; %r" % (types, instance))
-
-        def assertNotIsInstance(self, instance, types, msg=None):
-            if isinstance(instance, types):
-                self.fail(msg or "is an instance of %r; %r" % (types, instance))