I already looked for some information about it, and the only place with useful information is in the What’s New In Python 3.2 (use find to get to the place about ResourceWarning), it states:
A ResourceWarning is also issued when a file object is destroyed without having been explicitly closed. While the deallocator for such object ensures it closes the underlying operating system resource (usually, a file descriptor), the delay in deallocating the object could produce various issues, especially under Windows.