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