Use new way of specifying dev dependencies

This PR updates `pyproject.toml` to use new way of defining
development dependencies.

As shown in https://python-poetry.org/docs/managing-dependencies/
the new way (starting from Poetry 1.2.0) of defining
dev dependencies is by using `[tool.poetry.group.dev.dependencies]`.

According to `poetry` docs, they recommend migration
towards the new `group` notation.

Author: ivanovmg