update pytest config (#4463)

* update pytest config

pytest 3.8.0 was just released and officially deprecates some of the way
we do pytest marks. They introduced a new way to do this in 3.6 so this
PR switches to that mechanism and updates our minimum pytest requirement

* update the stubs

* also update wycheproof test config to remove deprecated paths

* don't need this any more
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 181d983..81656eb 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -32,7 +32,8 @@
     supported = pretend.stub(
         kwargs={"only_if": lambda backend: False, "skip_message": "Nope"}
     )
-    item = pretend.stub(keywords={"supported": [supported]})
+    node = pretend.stub(iter_markers=lambda x: [supported])
+    item = pretend.stub(node=node)
     with pytest.raises(pytest.skip.Exception) as exc_info:
         check_backend_support(True, item)
     assert exc_info.value.args[0] == "Nope (True)"
@@ -42,7 +43,8 @@
     supported = pretend.stub(
         kwargs={"only_if": lambda backend: True, "skip_message": "Nope"}
     )
-    item = pretend.stub(keywords={"supported": [supported]})
+    node = pretend.stub(iter_markers=lambda x: [supported])
+    item = pretend.stub(node=node)
     assert check_backend_support(None, item) is None