Some small comments:

* diff_bytes needs to be documented in the difflib docs and in Doc/whatsnew/3.5.rst.

* diff_bytes needs to be added to difflib.__all__

* This looks like a new feature to me, so it would be better to just commit it to the default branch.


+        except AttributeError:
+            raise TypeError('all arguments must be bytes, not %r' % s)

This could be changed to raise TypeError(...) from None


+            self.assertTrue(
+                isinstance(line, bytes),

assertIsInstance


+        try:
+            list(difflib.unified_diff(a, b, fna, fnb))
+            self.fail('expected TypeError')
+        except TypeError:
+            pass

with self.assertRaises(TypeError):
    list(difflib.unified_diff(a, b, fna, fnb))

looks more readable to me.