Remove use of six

[avelichka]

Removes the use of six so it is no longer a dependency and is not imported and used in any modules, tests, or scripts.

Fixes #1296

Description of the changes being introduced by the pull request:

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[jku]

Just two comments (unfortunately one touches a lot of lines):

• I believe import urllib does not bring urllib.request or urllib.parse into scope -- I think some dependency (requests?) maybe happens to import those so we don't notice this? If I'm correct, I'd be fine with import urllib.request style in tests so you don't need to modify all calls again but in the actual code some form of from urllib import x would probably be preferred for vendoring reasons
• for key, value in dict.items(mydict): is not wrong... but wouldn't for key, value in mydict.items(): be the normal way to do this? I hope there is a nice regex to do that replace if you end up doing it.

[avelichka]

Thank you for the review, @jku! Hopefully, I've managed to address your comments in the two new commits.

[jku]

Thanks, LGTM!

If you want to squash these into one commit and force-push feel free to do so, otherwise I'll do a "squash and merge" (either way is fine by me but in the latter case I can only mark you as co-author as the new commit will then be mine).

[avelichka]

I've squashed them into one commit. Thank you, @jku.