Add validation for backslashes in ZIP file names

[Xpirix]

Closes #254

Cc @lnicola

[lnicola]

We could also check for drive letters (D:foo.py is actually a valid path on Windows), but I think it's unlikely we'll ever see one in a real archive.

[Xpirix]

We could also check for drive letters (D:foo.py is actually a valid path on Windows), but I think it's unlikely we'll ever see one in a real archive.

Fixed it accordingly. Thanks @lnicola .