Issue a warning when a fixture named 'request' is collected

Fix #611

testing/deprecated_test.py

@@ -4,6 +4,8 @@ import os
 
 import pytest
 
+pytestmark = pytest.mark.pytester_example_path("deprecated")
+
 
 @pytest.mark.filterwarnings("default")
 def test_yield_tests_deprecation(testdir):
@@ -392,3 +394,13 @@ def test_pycollector_makeitem_is_deprecated():
     with pytest.warns(RemovedInPytest4Warning):
         collector.makeitem("foo", "bar")
     assert collector.called
+
+
+def test_fixture_named_request(testdir):
+    testdir.copy_example()
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "*'request' is a reserved name for fixtures and will raise an error in future versions"
+        ]
+    )