This should fix issue #968; this handles the situation in the following way:

If opening a file in a writeable mode a warning is raised and mmap is
disabled even if it was explicitly asked for.  This is done through a
new check that mmap.flush() is available--this check is performed on all
platforms and should require minimal overhead but we'll see... (the
check is only performed the first time an attempt is made to open a file
in writeable mode)
Added a note in the known issues page and a changelog entry
Fix syntax issue on Python 3
Add missing byte literal marker needed for Python 3 (since mkstemp automatically opens the file in binary mode)
Update known_issues.rst

Made more explicit that the Hurd mmap issue relates to FITS files specifically.
Update file.py

se -> set
Merge pull request #968 from iguananaut/issue-968

Test failure on Debian/Hurd