Fs.sshfs

Added
- Public `SSHFile` class in the the `fs.sshfs.file` to expose a file on
a remote SSH server obtained with `SSHFS.openbin`.
- Support for parameter extraction from FS URLs.
- `backports.configparser` dependency for Python < 3, to allow parsing
values from FS URLs parameters.

Changed
- Code used to manage the remote platform compatibility.

Fixed
- Use `gethostbyname` to the get the adress for an host adress in `SSHFS.__init__`.
- Make `convert_sshfs_errors` inherit from `AbstractContextManager` explicitly.

Fixed
- License file not being packed in wheel distributions.

Removed
- `enum34` optional dependency, originally required for Python < 3.4.
- Unneeded imports in the `fs.sshfs` submodules.

Fixed
- Test dependencies being unconditionally installed
(ReimarBauer [1](https://github.com/althonos/fs.sshfs/pull/1)).

Added
- Semantic version specifiers to dependencies in `setup.cfg`.

Fixed
- Potential bug occuring with `_SSHFileWrapper.truncate(0)`.
- `seek` and `truncate` methods of `_SSHFileWrapper` returning `None`.

Added
- Missing docstrings to `fs.sshfs` module members.
- `SSFS.geturl` implementation, allowing to retrieve an SSH URL
for the *download* purpose.
- Allow `SSHFS` to use SSH configuration values (from `~/.ssh/config` for instance)
if the given host is found in it.

Changed
- Make tests run against the local version of the code, allowing tests to
run without having to install the module first.
- Rewrite docstrings in Google style to be consistent with PyFilesystem2
documentation style.

Removed
- `SSHFS.getsyspath` implementation.