Windows fix: When PYTHONCASEOK is set, or for any other reason imports

Windows fix:  When PYTHONCASEOK is set, or for any other reason imports
are satisfied in a case-insensitive manner, the attempt to import (the
non-existent) fcntl gets satisfied by FCNTL.py instead, and the tempfile
module defines a Unix-specific _set_cloexec() function in that case.  As
a result, temp files can't be created then (blows up with an AttributeError
trying to reference fcntl.fcntl).  This just popped up in the spambayes
project, where there is no apparent workaround (which is why I'm pushing
this in now).