rewrote the initpkg mechanism and moved py lib implementation files to

_py/...  with py/__init__.py containing pointers into them

The new apipkg is only around 70 lines of code and allows
us to get rid of the infamous "py.__." by regular non-magical
"_py." imports. It is also available as a separately installable
package, see http://bitbucket.org/hpk42/apipkg

--HG--
branch : trunk

_py/test/plugin/pytest_assertion.py

@@ -0,0 +1,28 @@
+import py
+import sys
+
+def pytest_addoption(parser):
+    group = parser.getgroup("debugconfig")
+    group._addoption('--no-assert', action="store_true", default=False, 
+        dest="noassert", 
+        help="disable python assert expression reinterpretation."),
+
+def pytest_configure(config):
+    if not config.getvalue("noassert") and not config.getvalue("nomagic"):
+        warn_about_missing_assertion()
+        config._oldassertion = py.builtin.builtins.AssertionError
+        py.builtin.builtins.AssertionError = py.code._AssertionError 
+
+def pytest_unconfigure(config):
+    if hasattr(config, '_oldassertion'):
+        py.builtin.builtins.AssertionError = config._oldassertion
+        del config._oldassertion
+
+def warn_about_missing_assertion():
+    try:
+        assert False
+    except AssertionError:
+        pass
+    else:
+        py.std.warnings.warn("Assertions are turned off!"
+                             " (are you using python -O?)")