pyproject.toml: Update for Poetry 1.4/1.5

Can't install 1.2 now, and 1.3+ don't understand the old 1.2
`dev-dependencies` config option.

https://python-poetry.org/docs/1.4/managing-dependencies/#dependency-groups

poetry.lock

@@ -1196,4 +1196,4 @@ ci = ["coveralls"]
 [metadata]
 lock-version = "2.0"
 python-versions = "^3.7"
-content-hash = "fcbca8f2c0d936f5cd16cc3ffc308a2ebd933e596eacd457e6f92a5a1678e580"
+content-hash = "ad09ee67d43cf26190aba2e849b0059afa321e10d559afa041bdbb3a5fe3a7a3"

pyproject.toml

@@ -30,7 +30,7 @@ coveralls = {version = "^3.0.1", optional = true}
 [tool.poetry.extras]
 ci = ["coveralls"]
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 pytest = "^6.2.2"
 pytest-cov = "^2.11.1"
 pytest-asyncio = "^0.14.0"