astropy.io.fits defaults to memmap=True ?  #4196

Hi, I am working with fits files that contain a few thousand extensions (spectra) each. Looping over each extension causes my computer to run out of file handles and my code crashes with a "Too many open files" error. I think this is related to the memmap error in the warning box on the astropy.io.fits documentation:

"When opening a file with memmap=True, because of how mmap works this means that when the HDU data is accessed (i.e. hdul[0].data) another handle to the FITS file is opened by mmap. This means that even after calling hdul.close() the mmap still holds an open handle to the data so that it can still be accessed by unwary programs that were built with the assumption that the .data attribute has all the data in-memory."

I get the crash without setting memmap, and with setting memmap = True. It goes away with memmap=True and del hdu[ext].data or just setting memmap = False. It looks to me like I'm defaulting to memmap = True. Is that the correct behavior? If so, it should probably be noted more clearly in the documentation.