Fix/improve comparison of byte strings

Fixes https://github.com/pytest-dev/pytest/issues/5260.

changelog/5260.bugfix.rst

@@ -0,0 +1 @@
+Improve/fix comparison of byte strings with Python 3.

src/_pytest/assertion/util.py

@@ -263,8 +263,11 @@ def _compare_eq_sequence(left, right, verbose=0):
                 "At index {} diff: {!r} != {!r}".format(i, left[i], right[i])
             ]
             break
-    len_diff = len_left - len_right
 
+    if isinstance(left, bytes):
+        return explanation
+
+    len_diff = len_left - len_right
     if len_diff:
         if len_diff > 0:
             dir_with_more = "Left"

testing/test_assertion.py

@@ -331,6 +331,18 @@ class TestAssert_reprcompare:
         assert "- spam" in diff
         assert "+ eggs" in diff
 
+    def test_bytes_diff(self):
+        diff = callequal(b"spam", b"eggs")
+        if PY3:
+            assert diff == [
+                "b'spam' == b'eggs'",
+                "At index 0 diff: 115 != 101",
+                "Use -v to get the full diff",
+            ]
+        else:
+            # Handled as text on Python 2.
+            assert diff == ["'spam' == 'eggs'", "- spam", "+ eggs"]
+
     def test_list(self):
         expl = callequal([0, 1], [0, 2])
         assert len(expl) > 1