streamline docs, especially use "import pytest" and "pytest.*" in python code examples instead of "import py" and "py.test.*".

testing/test_helpconfig.py

@@ -43,8 +43,8 @@ def test_hookvalidation_unknown(testdir):
 
 def test_hookvalidation_optional(testdir):
     testdir.makeconftest("""
-        import py
-        @py.test.mark.optionalhook
+        import pytest
+        @pytest.mark.optionalhook
         def pytest_hello(xyz):
             pass
     """)