Run pyupgrade, flynt, autoflake, remove six dependency

[akx]

Description

The feature or problem addressed by this PR

The repo could use modern (Python 3.6+) features more.

What your changes do and why you chose this solution

Hot on the heels of #873, which switched the repo to modern formatting...

Ran three mechanical tools:

• pyupgrade to fix "legacy Pythonisms"
• autoflake to remove unused imports (either left behind by pyupgrade, or otherwise unused)
• flynt, to fix non-f-string string formatting
• flynt -tc, to fix string concatenation too (which is more intrusive and could be unsafe, but tests should catch things)

Then followed up with some manual cleaning.

Checklist

• Checked that no other issues or pull requests exist for the same issue/change
• Added tests covering the new functionality
• Updated documentation OR the change is too minor to be documented
• Updated CHANGELOG.md OR changes are insignificant

[c00kiemon5ter]

Hello and thanks for this. I merged another PR so this now has a few conflicts but should be super easy to fix. I will try to go through this and merge asap.
Thanks for pointing out new tools, maybe those can become part of the linting/re-formatting processes we have.

[akx]

@c00kiemon5ter I can take a look at the conflicts, no worries!

In case you're interested, I'm invested in this repo since the other SAML toolkit for Python seems to be going the way of the dodo... SAML-Toolkits/python3-saml#320

[akx]

Oh and re

Thanks for pointing out new tools, maybe those can become part of the linting/re-formatting processes we have.

yep – these are super easy to integrate with pre-commit :)

[c00kiemon5ter]

Thank you @akx - this is now merged.