When calling `shutil.copy('file.txt', 'not_here/')`,
if directory `not_here/` does not exist,
the raised error is:

    IsADirectoryError: [Errno 21] Is a directory: 'not_here/'

If the intent of the user was to copy a file in a directory
but the user forgot to create the destination directory,
this can be very misleading,
as the error tends to indicate that the directory exists.

It happened to me and I was thinking
"yes it's a directory, then what?
that's exactly what I want,copy to this directory!"
when the problem was that I forgot to create the destination directory.

I would suggest to catch the `IsADirectoryError`
in shutil.copyfile() (at `open(dst, 'wb')`)
and raise instead an error saying something like
"destination is a directory AND this directory does not exists".