a) is #14013; b) is #14011.

If Lars agrees to #14013, I will withdraw this patch in favor of another one that would make shutil automatically support all compressors that tarfile supports (my b) point).

Note that this is not going to be pain-free, as for example bzip2 compression is obtained with the strings “bz2”, “bzip2”, “bzip” or “bztar” depending on the modules; I think it’s worth it nonetheless, even if I have to add an ugly conversion in shutil for compat.