Add test for python .pyc/.pyo rewriting

This catches regressions in the fix in the previous commit, where old
stale .py[oc] files can wrongly become "unstale" when we change the
.py file mtime to 1.

tests/testpython.py

@@ -0,0 +1,4 @@
+#!/usr/bin/python2
+
+import importme
+print importme.foo