Update config and drop python < 3.8

[MegaIng]

This PR consists of 2 parts:

It removes python code specific to python 3.6/3.7 (and some only relevant for 2.7 that we forgot!)

It updates quite a bit config, although it's little "real" changes. I list them here as checkboxes so that you @erezsh can check them off if you agree with them

• Drop 3.6 and 3.7 from all relevant locations. Note that there is now the sentence Since version 1.2, only Python versions 3.8 and up are supported, so we should increment the version when this is released.
• Add 3.13-dev to the tests workflow
• Change pypy-3.7 to pypy-3.10 in the tests workflow.
• Move pytest.ini into pyproject.toml, with the goal of reducing the number of config files we have. I didn't do the same for tox.ini because it would just be putting the raw .ini file into a multiline string, which seems too weird for me.
• Added unused-ignore to the error codes for mypy which tells us when we can drop an ignore (I also dropped a few ignores based on this warning)
• Remove the pin for the types-regex version. This is no longer necessary with us dropping python <3.8
• Pin mypy to 1.10 instead of 0.950. Pinning is probably a good idea in general, but everything works with the current version, so no reason I know to keep an older version.
• Change fail_ci_if_error in codecov.yml from true to false. This is primarily a selfish change, since it's very annoying to always get failed CI on my repo since I don't have the secret set. If you don't like this version, there is also a way to conditionally execute the workflow which would be an alternative solution.

[erezsh]

Looks good!

As for codecov, ideally I would like it to work for everyone. I have the CODECOV_TOKEN env var set for the repo. Do you have any idea why it doesn't work for you?

Also, I think the version bump commit should be part of this PR. Feel free to do it, or I can create and push it here if you prefer.

[MegaIng]

I have the CODECOV_TOKEN env var set for the repo. Do you have any idea why it doesn't work for you?

Because quite reasonably, it's a secret so that it's not publicly accessible, and to specifically prevent everyone from pushing stuff to codecov.

[erezsh]

Well, I'd still like to fix codecov for all pull requests, but I suppose it doesn't have to be in this PR.

So, is it ready for merge?

[MegaIng]

Ok, looks like the codecov action is currently a bit buggy: https://github.com/codecov/codecov-action/issues I am going to revert these commits and we can try again in a few weeks.

[MegaIng]

Now we are ready to merge.