Deprecate calling fixture functions directly

This will now issue a RemovedInPytest4Warning when the user calls
a fixture function directly, instead of requesting it from test
functions as is expected

Fix #3661

testing/example_scripts/tmpdir/tmpdir_fixture.py

@@ -0,0 +1,7 @@
+import pytest
+
+
+@pytest.mark.parametrize("a", [r"qwe/\abc"])
+def test_fixture(tmpdir, a):
+    tmpdir.check(dir=1)
+    assert tmpdir.listdir() == []