Return the image part in this collection having a SHA1 hash matching
        *sha1*, or |None| if not found.

def _get_by_sha1(self, sha1):
        """
        Return the image part in this collection having a SHA1 hash matching
        *sha1*, or |None| if not found.
        """
        for image_part in self._image_parts:
            if image_part.sha1 == sha1:
                return image_part
        return None