mkdir broken for adlfs

[dtrifiro]

In #83, mkdir(parents=True) switched from using self._accessor.mkdir to self._accessor.makedirs.

For some implementations (such as adlfs and gcsfs, possibly more), makedirs is not implemented, causing calling upath.mkdir to be a no-op, whereas it previously worked to create containers/buckets.

See dtrifiro@fac2bcf on https://github.com/dtrifiro/universal_pathlib/tree/broken-mkdir-examples