Work around a bug in setting and getting the mtime in python 2 See http://bugs.python.org/issue12904. Basically, we can get the mtime in nanosecond precision, but only set it in microsecond precision. This means that the shutil.copy2 will not set the destination's mtime to exactly the same mtime as our source. The end result is that we can *always* end up copying the extension because the source always appears newer. We add a microsecond of fudge time when checking to see if the source is newer than the destination to get around this. This bug is fixed in Python 3.3+, I believe.