Refactor direct fixture call warning to avoid incompatibility with plugins

This refactors the code so we have the real function object right during
collection. This avoids having to unwrap it later and lose attached information
such as "async" functions.

Fix #3747

testing/python/fixture.py

@@ -1470,10 +1470,9 @@ class TestFixtureManagerParseFactories(object):
                     print (faclist)
                     assert len(faclist) == 3
 
-                    kwargs = {'__being_called_by_pytest': True}
-                    assert faclist[0].func(item._request, **kwargs) == "conftest"
-                    assert faclist[1].func(item._request, **kwargs) == "module"
-                    assert faclist[2].func(item._request, **kwargs) == "class"
+                    assert faclist[0].func(item._request) == "conftest"
+                    assert faclist[1].func(item._request) == "module"
+                    assert faclist[2].func(item._request) == "class"
         """
         )
         reprec = testdir.inline_run("-s")