synchronous `rmdir()` fails silently

[isoflavone-r]

Hello.

Trying to remove directories using SSHFileSystem via rmdir fails silently.

Appears to be missing the synchronous wrapper for _rmdir. i.e. the equivalent of: mkdir = sync_wrapper(_mkdir) and thus ends up all the way in AbstractFileSystem.rmdir which is implemented as pass # not necessary to implement, may not have directories.

A local test of adding the sync_wrapper works ok so far.

[efiop]

Hey @isoflavone-r , thanks for the report! Makes sense. Would you be interested in contributing a fix? If so, please feel free to submit a PR.