Fix compatibility with pytest 8.4

pytest-dev/pytest#12473 removed
`_pytestfixturefunction`.  The `getfixturemarker` function is still
private, but has existed across all pytest versions that we support.

Fixes: bitprophet#33

Author: cjwatson

pytest_relaxed/classes.py

@@ -6,6 +6,7 @@
 
 # NOTE: don't see any other way to get access to pytest innards besides using
 # the underscored name :(
+from _pytest.fixtures import getfixturemarker
 from _pytest.python import PyCollector
 
 
@@ -27,8 +28,7 @@ def istestfunction(obj, name):
         "teardown",
         "teardown_method",
     )
-    # TODO: is this reliable? how about __pytest_wrapped__?
-    is_fixture = hasattr(obj, "_pytestfixturefunction")
+    is_fixture = getfixturemarker(obj) is not None
     return not (is_hidden_name or is_fixture)