QA and CI: Format code using ruff. Validate using ruff and mypy.

[amotl]

About

• Run the ruff code formatter and linter on the code base, and the mypy type checker.
• Invoke the "lint" job also on CI.

Rationale

Before releasing 1.0.0, we would like to get the shape of the repository in order. This patch is one in a series to support that.

Details

No other functional changes inside.

References

• Testing: Refactor software tests into dedicated directory tests #659

[kneth]

Reformatting always gives you huge PRs.

[kneth]

Only if you will catch the exception where you call _timezone_from_utc_offset

[amotl]

On this spot, I am referring to the fact that the ruff linter discourages the use of assert statements in code:

S101 Use of `assert` detected

-- https://docs.astral.sh/ruff/rules/assert/

That's why it needs to be suppressed now, alongside a few other spots in this patch, using # noqa: S101.

My proposal would be to rewrite the assert statements to constructs like that:

if len(tz) != 5:
    raise ValueError(f"Time zone '{tz}' is given in invalid UTC offset format")

I think that would be the correct approach to modernize the code base. wdyt?

[kneth]

Now it makes sense, and I agree that raising an exception is a better option.