simplify rewrite-on-import

Use load_module on the import hook to load the rewritten module. This allows the
removal of the complicated code related to copying pyc files in and out of the
cache location. It also plays more nicely with parallel py.test processes like
the ones found in xdist.

_pytest/assertion/__init__.py

@@ -28,7 +28,6 @@ class AssertionState:
     def __init__(self, config, mode):
         self.mode = mode
         self.trace = config.trace.root.get("assertion")
-        self.pycs = []
 
 def pytest_configure(config):
     mode = config.getvalue("assertmode")
@@ -62,8 +61,6 @@ def pytest_configure(config):
     config._assertstate.trace("configured with mode set to %r" % (mode,))
 
 def pytest_unconfigure(config):
-    if config._assertstate.mode == "rewrite":
-        rewrite._drain_pycs(config._assertstate)
     hook = config._assertstate.hook
     if hook is not None:
         sys.meta_path.remove(hook)
@@ -77,8 +74,6 @@ def pytest_collection(session):
         hook.set_session(session)
 
 def pytest_sessionfinish(session):
-    if session.config._assertstate.mode == "rewrite":
-        rewrite._drain_pycs(session.config._assertstate)
     hook = session.config._assertstate.hook
     if hook is not None:
         hook.session = None