Only consider actual functions when considering hooks

Fix #3775

src/_pytest/config/__init__.py

@@ -1,6 +1,7 @@
 """ command line options, ini-file and conftest.py processing. """
 from __future__ import absolute_import, division, print_function
 import argparse
+import inspect
 import shlex
 import traceback
 import types
@@ -252,6 +253,10 @@ class PytestPluginManager(PluginManager):
         method = getattr(plugin, name)
         opts = super(PytestPluginManager, self).parse_hookimpl_opts(plugin, name)
 
+        # consider only actual functions for hooks (#3775)
+        if not inspect.isroutine(method):
+            return
+
         # collect unmarked hooks as long as they have the `pytest_' prefix
         if opts is None and name.startswith("pytest_"):
             opts = {}