Ssh2-python3

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Changes
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* Implemented SCP send and recv methods, all versions.
* Conditional compilation of features requiring newer versions of libssh2.
* Implemented channel receive window adjust, x11_*, poll and handle extended data methods.
* Implemented session get/set blocking, get/set timeout.
* Updated agent connection error exception name.
* Renamed session method name to match libssh2.
* Info extension classes for SCP file stat structure.

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Changes
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* Added context manager to SFTP handle
* Implemented SFTP write, seek, stat, fstat and last_error methods.
* Implemented SFTPAttribute object creation and de-allocation - added unit test.

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Changes
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* Updated API
* Updated session, channel, agent and pkey to accept any string type arguments.
* Added get_exit_signal implementation for channel.