Fix: zip can contain directories and files at the root

[jteulade]

Choose a zip with a file XXX and a directory at the root of the archive.
Try to use extract_file method, you get this error:
[Errno 21] Is a directory: 'XXX'
We should not consider the file as a directory.

[remi-braun]

Could you add:

• a CI test (we'll see together next week)
• a line in CHANGES.md about this bugfix

[remi-braun]

Could you detail a bit how your change makes the directory check working?
A could you skip a line before the comment? ;)