test: fix pytest 4 markers usage

A getattr on the function doesn't work since pytest 4.
Looks the correct way to do this is to test the presence of "pytestmark"
attribute on the function which is either undefined or contains a list
of Marker objects.
We don't care about backward compat with older pytest versions for
testinfra own tests.

test/conftest.py

@@ -158,8 +158,10 @@ def host(request, tmpdir_factory):
     image, kw = parse_hostspec(request.param)
     spec = BaseBackend.parse_hostspec(image)
 
-    if getattr(request.function, "destructive", None) is not None:
-        scope = "function"
+    for marker in getattr(request.function, 'pytestmark', []):
+        if marker.name == 'destructive':
+            scope = "function"
+            break
     else:
         scope = "session"
 
@@ -234,20 +236,13 @@ def docker_image(host):
 
 def pytest_generate_tests(metafunc):
     if "host" in metafunc.fixturenames:
-        # Supported in pytest 3.6+.  Earlier versions need to use the
-        # MarkInfo object directly as in the else: clause; this is
-        # marked for removal in pytest4.
-        if hasattr(metafunc, 'definition'):
-            marker = metafunc.definition.get_closest_marker(
-                "testinfra_hosts")
-        else:
-            marker = getattr(metafunc.function, "testinfra_hosts", None)
-        if marker is not None:
-            hosts = marker.args
+        for marker in getattr(metafunc.function, 'pytestmark', []):
+            if marker.name == 'testinfra_hosts':
+                hosts = marker.args
+                break
         else:
             # Default
             hosts = ["docker://debian_stretch"]
-
         metafunc.parametrize("host", hosts, indirect=True,
                              scope="function")