catch AttributeError in cleanup_connection_files

can be triggered during garbage collection (os.remove is destroyed before the KM instance)
skip remaining ipc test on Windows

ipc is not a valid transport on Windows.
Merge pull request #2904 from minrk/winipc

Skip remaining IPC test on Windows

also fix an issue that could appear as an AttributeError when KM.__del__ is called during gc on process exit.

closes #2898