Fix warnings with attrs 19.2 and fix object assertions

attrs 19.2 deprecated cmp in favor of the dataclass-ish eq/order duo.

This causes deprecation warnings that in turn break some of the cool new deep
object comparisons. Since we at attrs expected this to be a problem, it shipped
with helpers to write backward and forward compatible code.

This PR uses that and avoids changed to minimal versions.

testing/test_assertion.py

@@ -9,6 +9,7 @@ import pytest
 from _pytest import outcomes
 from _pytest.assertion import truncate
 from _pytest.assertion import util
+from _pytest.compat import attrs_no_eq
 
 
 def mock_config():
@@ -687,7 +688,7 @@ class TestAssert_reprcompare_attrsclass:
         @attr.s
         class SimpleDataObject:
             field_a = attr.ib()
-            field_b = attr.ib(cmp=False)
+            field_b = attr.ib(**attrs_no_eq)
 
         left = SimpleDataObject(1, "b")
         right = SimpleDataObject(1, "b")