- The :class:`.ScriptDirectory` system that loads migration files

- The :class:`.ScriptDirectory` system that loads migration files
from a  ``versions/`` directory now supports so-called
"sourceless" operation,  where the ``.py`` files are not present
and instead ``.pyc`` or ``.pyo`` files are directly present where
the ``.py`` files should be.  Note that while Python 3.3 has a
new system of locating ``.pyc``/``.pyo`` files within a directory
called ``__pycache__`` (e.g. PEP-3147), PEP-3147 maintains
support for the "source-less imports" use case, where the
``.pyc``/``.pyo`` are in present in the "old" location, e.g. next
to the ``.py`` file; this is the usage that's supported even when
running Python3.3.  #163