Fs.sshfs

Changed
- Add explicit support for Paramiko 3 ([57](https://github.com/althonos/fs.sshfs/issues/57)).

Fixed
- `SSHFS.move` not supporting the `preserve_time` argument ([51](https://github.com/althonos/fs.sshfs/issues/51)).

Added
- `SSHFS.upload` (resp. `download`) implementation using `paramiko.SFTP.putfo`
(resp. `getfo`).
- This changelog file.

Changed
- Switched from Travis-CI and AppVeyor to GitHub Actions for continuous integration.
- Rewrote `README.rst` in Markdown format.
- Mark the project as *Stable* in `setup.cfg` classifiers.

Fixed
- Make `SSHFile` record the mode it was created with.

Added
- `semantic-version` test requirement.
- `SSHFS.islink` method with behaviour matching `OSFS.islink`.
- Additional documentation of constructor arguments in `SSHFS` docstring.
- Independent timeout for arbitrary SSH commands ([39](https://github.com/althonos/fs.sshfs/issue/39)).
- Support for passing arbitrary `paramiko.MissingHostKeyPolicy` to `SSHFS`.

Changed
- `SSHFS.getinfo` now follows symlinks like `OSFS.getinfo`.

Added
- Explicit support for Python 3.8.

Fixed
- Issue with `SSHFS.close` when exiting while an `SSHFS` instance is still open.