[#25] Add putty v3 support. Get the code working on py3.

[adiroiban]

Scope

Fixes #25
Fixes #54

This is to help with the migration of chevah/server to py3

It started as support for both py2 and py3, but we should merge this will all tests passing and with only support for py3.

Changes

Move the package to a non-namespace.
Modernize the project build with the code in src and with setup.cfg

How to try and test the changes

reviewers: @danuker

More of a FYI.

Most of the code is tested in chevah/server

We have a big tech-debt here and we need to rewrite the tests...

or redesing the project/code so that the low-level sign/verify is used directly from Twisted, without replication

And chevah-keycert will only handle the serialization/deserialization of various SSH keys.

There is also the idea of moving all this code into the python-cryptography code, as that project already has basic support for OpeSSH key format.

and chevah-keycert uses cryptography for low-level operations

[adiroiban]

Dan, this is ready for review.
I plan to integrate the code in chevah/server to get support for Putty v3

SOme of the code from here is already used in chevah/server ... but I didn't had time to release chevah-keycert

Now, with the test passing, I think this is ready to merge.

The putty v3 code is a lot of copy paste... but we can improve that in a separate ticket

needs-review

[adiroiban]

@danuker it looks like this repo is not handled by chevah-robot ?

[adiroiban]

I am merging this as the code is in used https://github.com/chevah/server/pull/6632

for any issues we can open a new PR