Move lru_cache wrapper to compat

Ref: https://github.com/pytest-dev/pytest/pull/4227#discussion_r228060373

src/_pytest/compat.py

@@ -417,3 +417,16 @@ class FuncargnamesCompatAttr(object):
     def funcargnames(self):
         """ alias attribute for ``fixturenames`` for pre-2.3 compatibility"""
         return self.fixturenames
+
+
+if six.PY2:
+
+    def lru_cache(*_, **__):
+        def dec(fn):
+            return fn
+
+        return dec
+
+
+else:
+    from functools import lru_cache  # noqa: F401

src/_pytest/config/__init__.py

@@ -1,7 +1,6 @@
 """ command line options, ini-file and conftest.py processing. """
 from __future__ import absolute_import, division, print_function
 import argparse
-import functools
 import inspect
 import shlex
 import types
@@ -20,6 +19,7 @@ import _pytest.hookspec  # the extension point definitions
 import _pytest.assertion
 from pluggy import PluginManager, HookimplMarker, HookspecMarker
 from _pytest._code import ExceptionInfo, filter_traceback
+from _pytest.compat import lru_cache
 from _pytest.compat import safe_str
 from .exceptions import UsageError, PrintHelp
 from .findpaths import determine_setup, exists
@@ -894,6 +894,7 @@ class Config(object):
             assert type is None
             return value
 
+    @lru_cache(maxsize=None)
     def _getconftest_pathlist(self, name, path):
         try:
             mod, relroots = self.pluginmanager._rget_with_confmod(name, path)
@@ -908,10 +909,6 @@ class Config(object):
             values.append(relroot)
         return values
 
-    if six.PY3:
-        # once we drop Python 2, please change this to use the normal decorator syntax (#4227)
-        _getconftest_pathlist = functools.lru_cache(maxsize=None)(_getconftest_pathlist)
-
     def _get_override_ini_value(self, name):
         value = None
         # override_ini is a list of "ini=value" options