Modernize the code for Python 3.8

[ulgens]

With the help of pyupgrade

Small cleaning before removing Python 3.8 support (EOL)

[ulgens]

@Nusnus This will help with 3.8 removal and #1461

[Nusnus]

I wonder if we should align with python 3.10 already as 3.9 is EOL too.

[ulgens]

@Nusnus I can create other PRs to continue with this work. I think it's better to keep them separete and not to risk have huge PRs that hard to navigate.

[Nusnus]

@Nusnus I can create other PRs to continue with this work. I think it's better to keep them separete and not to risk have huge PRs that hard to navigate.

Sounds great! Thank you!

[ulgens]

@Nusnus Thank you.

Also, about the CI failures,

flower/events.py:18:0: C0103: Variable name "PROMETHEUS_METRICS" doesn't conform to snake_case naming style (invalid-name)

This doesn't come from the changes done in the PR and I didn't care about it much - I'm not even sure why it expects a snake_case naming for a module level variable name. If you have any recommendations, I can handle that in a separate PR.