remove DSS key references; upgrade paramiko dependency to version 4.0

[rawwar]

Raising this PR as Apache Airflow uses this package. We wanted to bump paramiko>=4.0.0(PR). But, found that sshtunnel uses DSS Key. I did see earlier PR's. But, they were failing tests. This is my attempt to try to fix them.

• [x] removed DSSKey usage
• [x] updated min. version of paramiko to 4.0
• [x] updated min. python version to 3.9
• [x] dropped support for python 2.x
• [x] fixed failures of docs tests in circleci
• [x] updated changelog, docs
• [ ] updated appveyor config - This is still failing. Not sure how to fix

Note: I've used copilot

[coveralls]

coverage: 91.704% (+0.04%) from 91.667% when pulling b93a5e91dc164c31dacf7201a9b56c064b0fd1f6 on rawwar:kalyan/bump_paramiko into dc0732884379a19a21bf7a49650d0708519ec54f on pahaz:master.

[rawwar]

Looks like, its not possible to enable rolling build from appveyor.yml config. it needs to be done via UI - https://www.appveyor.com/docs/build-configuration/#rolling-builds

Appveyor builds are running for every commit. I think, we don't need the older builds running when the PR is updated . CC: @pahaz

[rawwar]

@pahaz , I did make a lot of changes in the PR assuming too many things. Appreciate your feedback and how we want to resolve this.

[rawwar]

Also, for Appveyor config, do you know what will be the python path? I did some debugging and still was unable to figure it out. I did try adding more debug logs.

[wenderson-cmp]

any updates ? @pahaz

[lozik4]

Maybe release 0.4.1 where bamp paramico <=3.5.1 After that, release 0.5.0 without support DSSkey @pahaz ?