raise PR later: fs.zipfs won't close properly

[chfw]

import fs
fs.open_fs('zip://I_do_not_exist_.zip')

Exception ignored in: <function FS.del at 0x10491cd08>
Traceback (most recent call last):
File "/Users/jaska/github/moremoban/moban-env/lib/python3.7/site-packages/fs/base.py", line 117, in del
self.close()
File "/Users/jaska/github/moremoban/moban-env/lib/python3.7/site-packages/fs/zipfs.py", line 437, in close
self._zip.close()
AttributeError: 'xxx' object has no attribute '_zip'

[chfw]

PyFilesystem/pyfilesystem2#333

[chfw]

PyFilesystem/pyfilesystem2#330