[MAINT] Combine the two dev deps sections for poetry

[rgbkrk]

Due to poetry changes we now have two development dependencies sections:

[tool.poetry.dev-dependencies]
flake8-docstrings = "^1.6.0"
pytest = "^7.2.2"
black = "^23.1.0"
isort = "^5.10.1"
pytest-cov = "^4.0.0"
pytest-asyncio = "^0.19.0"
nox = "^2022.1.7"
nox-poetry = "^1.0.1"
pytest-mock = "^3.8.2"
bump2version = "^1.0.1"

[tool.poetry.group.dev.dependencies]
pandas = "^1.5.3"

Those need to all be under tool.poetry.group.dev.dependencies based on the feedback poetry now gives when you poetry add --dev

The --dev option is deprecated, use the `--group dev` notation instead.