Fs.sshfs

Added
- Dedicated implementation of `SSHFS.scandir` using `paramiko.SFTP.listdir_attr`
(jwnimmer-tri [37](https://github.com/althonos/fs.sshfs/pull/37)).

Fixed
- Assume *unknown* platform is `_exec_command` throws an exception
(mrk-its [35](https://github.com/althonos/fs.sshfs/pull/35)).

Added
- Explicit support for Python 3.7.
- Section documenting the support of `sftp` FS URLs to `README.rst`.
- `support_rename` key to the `SSHFS.getmeta` options.
- `SSHFS.move` implementation using the `paramiko.SFTP.rename` function.

Fixed
- Typos in `README.rst`
(jayvdb [28](https://github.com/althonos/fs.sshfs/pull/28),
timnyborg [32](https://github.com/althonos/fs.sshfs/pull/32)).

Changed
- Replace `cached-property` dependency with improved `property-cached` fork.

Added
- `prefetch` argument to `SSHFS.openbin` to enable prefetching files when
they are open in reading mode.

Added
- Explicit support for `fs ~=2.2`.