Drop Python 3.7, 3.8, 3.9 support

[ivanovmg]

This PR drops support for Python 3.7, 3.8 and 3.9 versions.

Here we updated python version in pyproject.toml and then updated poetry.lock file as described below (please correct me if it is totally wrong way of doing things).

Update to poetry.lock file:

1. Bump python version to 3.10+ (previous commit)
2. Re-build poetry.lock file as shown below:

$ rm poetry.lock $ poetry install 

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

Format is:

• ClosesDrop Python 3.7 through 3.9 support #496

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[sobolevn]

Please, take a look at other sys.version_info checks

[sobolevn]

You can remove that now, because there are no python versions that won't execute this branch.

[sobolevn]

you can use pip-check lib from PyPI to find outdate packages and update them all at once in pyproject.toml

[sobolevn]

I pushed a fix for this problem to master, please, rebase

[ivanovmg]

I pushed a fix for this problem to master, please, rebase

OK, thanks! I tried to fix it myself, but now I will rebase

[ivanovmg]

It turns out that there are multiple mypy check failures with the newest mypy version.
I tried to fix some of them, but it goes beyond my experience and knowledge of type checking.

@sobolevn please suggest the way forward.
I suppose that some of them can be silenced, considering a dynamic nature of the typeclass implementation, but most of them will still have to be handled somehow.

[sobolevn]

You can use older mypy for now, I will fix these issues later :)

[ivanovmg]

You can use older mypy for now, I will fix these issues later :)

I reverted back to mypy v0.942, but surprisingly the errors persist (even more errors appeared).

Update

I noticed that when I drop strict=true in setup.cfg, then mypy checks are successful, but then flake8 complains.

With strict = true we have multiple mypy failures.

[sobolevn]

You can delete nitpick section and dependency from pyproject.toml