is_dir does not handle cdir and pdir extensions

[RFC3659](https://www.rfc-editor.org/info/rfc3659/#section-7.5.1) gives the following types of directories that can be returned by the FTP protocol:
* `cdir`: the listed directory
* `pdir`: a parent directory
* `dir`: a directory or sub-directory

aioftp `is_dir` only returns `True` when the returned string is `dir`. We discovered that when a FTP server started returning `cdir` instead, making `is_dir` return an unexpected result.

Could it be possible to make `is_dir` recognize the values `cdir` and `pdir`?