Add support for SSHAgent and Key Types outside of RSA

[calebstewart]

Description of Changes

This PR overhauls the SSH channel implementation to more correctly utilize Paramiko, and also moves our dependency from paramiko to paramiko-ng, which provides fixes for previously broken parts of paramiko.

This not only simplifies the SSH channel implementation, but also means that SSH-Agent should work out of the box and that you can now use keys aside from RSA (tested with an ed25519 key).

Fixes #91.

Major Changes Implemented:

• Replaced paramiko with paramiko-ng
• Utilized Paramiko SSHClient which will also utilize the SSHAgent if available by default and supports key types aside from RSA (#91)

Pre-Merge Tasks

• Formatted all modified files w/ python-black
• Sorted imports for modified files w/ isort
• Ran flake8 on repo, and fixed any new problems w/ modified files
• Ran pytest test cases
• Added brief summary of updates to CHANGELOG (under [Unreleased])

For issues with pre-merge tasks, see CONTRIBUTING.md