Fixes failing tests because of breaking API changes of pytest 4.

https://bugs.gentoo.org/675838

Signed-off-by: Andrew Attali <me@aphypnise.eu>

diff --git a/tests/conftest.py b/tests/conftest.py
index b2fc766..3d09715 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -42,7 +42,7 @@ def no_cache(monkeypatch):
 
 @pytest.fixture(autouse=True)
 def functional(request):
-    if request.node.get_marker('functional') \
+    if request.node.get_closest_marker('functional') \
             and not request.config.getoption('enable_functional'):
         pytest.skip('functional tests are disabled')