Add initial testing foundation

[NotTheEvilOne]

This PR adds the initial testing foundation:

• pytest
• paramiko based mock SSH server

Closes #14

[NotTheEvilOne]

Additional note: I do not agree with all formatting rules applied. The still marked "formatting error" for the ternary operator does not make sense at all and mixes condition with return value for one but not both possibilities.

[brueggemann]

Additional note: I do not agree with all formatting rules applied. The still marked "formatting error" for the ternary operator does not make sense at all and mixes condition with return value for one but not both possibilities.

I don't really know, which rule was applied on that line, but I think, probably the line was to long with the else statement in the first line, so it did the line break before else?

In my opinion, the else in the second line is better to read, because now it is more visible, that the AUTH_FAILED is part of the ternary the line before

[brueggemann]

Please also use type hints for method arguments and return values