Parallel-ssh

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Fixes
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* Native client SCP and SFTP uploads would not handle partial writes from waiting on socket correctly.
* Native client ``copy_file`` SFTP upload would get stuck repeating same writes until killed when copying multi-MB
files from Windows clients - 148
* Native client ``scp_send`` would not correctly preserve file mask of local file on the remote.
* Native client tunnel, used for proxy implementation, would not handle partial writes from waiting on socket correctly.

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Changes
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* Removed libssh2 native library dependency in favour of bundled ``ssh2-python`` libssh2 library.
* Changed native client forward agent default behaviour to off due to incompatibility with certain SSH server
implementations.
* Added keep-alive functionality to native client - defaults to ``60`` seconds.
``ParallelSSHClient(<..>, keepalive_seconds=<interval>)`` to configure interval. Set to ``0`` to disable.
* Added ``~/.ssh/id_ecdsa`` default identity location to native client.

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Fixes
------

* Native parallel client ``forward_ssh_agent`` flag would not be applied correctly.

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Fixes
------

* Native client socket timeout setting would be longer than expected - 133

Packaging
---------

* Added Windows 3.7 wheels

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Changes
--------

* Native client no longer requires public key file for authentication.
* Native clients raise ``pssh.exceptions.PKeyFileError`` on object initialisation if provided private key file paths
cannot be found.
* Native clients expand user directory (``~/<path>``) on provided private key paths.
* Parallel clients raise ``TypeError`` when provided ``hosts`` is a string instead of list or other iterable.

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Changes
--------

* Better tunneling implementation for native clients that supports multiple tunnels over single SSH connection for
connecting multiple hosts through single proxy.
* Added ``greenlet_timeout`` setting to native client ``run_command`` to pass on to getting greenlet result to allow
for greenlets to timeout.
* Native client raises specific exceptions on non-authentication errors connecting to host instead of generic
``SessionError``.


Fixes
------

* Native client tunneling would not work correctly - 123.
* ``timeout`` setting was not applied to native client sockets.
* Native client would have ``SessionError`` instead of ``Timeout`` exceptions on timeout errors connecting to hosts.