Add mypy lint group dependency

[stankudrow]

Introducing the mypy type analyser under the lint dependency group alongside to the updated ruff linter.

The py.typed marker is placed into the package. Despite type stubs are not shipped yet, this marker lays the first stone.

Some minor sanity work was done for connection, errors and pool modules.

Thanks to this article for brief yet clean examples.

UPD: related to the issue #126 .

[stankudrow]

@long2ice , good day.

Can this PR go first, and then either #129 or #130 ?
The mypy library will enhance linting and code quality, also will be incorporated into the project CI/CD.

Thank you.

[stankudrow]

@long2ice , hello.

I have not addressed all mypy complaints, but the first brick is laid. If coping with issues piece by piece in every PR, the type hint problems may eventually disappear without lots of files to review at once.

A possible sequence: this PR -> #129 -> #130 and then a new release and closing the issues: #126, #121 and #127 .

[stankudrow]

@long2ice , hello.

Is this PR is OK to be merged?

[stankudrow]

@long2ice , hello.

The PR is rebased, if you don't mind, let's merge it before the PR #130 ?

[long2ice]

Thanks!