Create a hard link to a file.
If either src_dir_fd or dst_dir_fd is not None, it should be a file
descriptor open to a directory, and the respective path string (src or
dst) should be relative; the path will then be relative to that
directory.
If follow_symlinks is False, and the last element of src is a symbolic
link, link will create a link to the symbolic link itself instead of the
file the link points to.
src_dir_fd, dst_dir_fd, and follow_symlinks may not be implemented on
your platform. If they are unavailable, using them will raise
a NotImplementedError.