Update tox.ini for poetry

tox.ini

@@ -8,11 +8,8 @@ isolated_build = True
 
 [testenv]
 description = Run unit-testing
-# develop temporary disabled as project packaging does not work with it yet:
-# https://github.com/willmcgugan/rich/issues/345
-usedevelop = False
 deps =
-    -r requirements-dev.txt
+    poetry
 # do not put * in passenv as it may break builds due to reduced isolation
 passenv =
     CI
@@ -25,8 +22,7 @@ setenv =
     PYTHONDONTWRITEBYTECODE=1
     PYTHONUNBUFFERED=1
 commands =
-    # failsafe as older pip may install incompatible dependencies
-    pip check
+    poetry install
     pytest --cov-report term-missing --cov=rich tests/ {posargs}
 
 [testenv:lint]