Ensure `__file__` exists (and possibly `__cached__`)#72

Fixed ----- * Ensure that modules have a ``__file__`` attribute, and possibly a ``__cached__`` attribute. The Python data model docs make it clear that these attributes aren't guaranteed, but it is nevertheless common for authors to assume that ``__file__`` exists. The string value may not be usable for a specific purpose, but the attributes now exist with the correct type for increased compatibility.