bugfix: use extras in tox to ensure installation of test deps

Author: RonnyPfannschmidt

tox.ini

@@ -9,12 +9,7 @@ ignore=E203,W503
 
 [testenv]
 usedevelop=True
-deps=
-    pytest
-    setuptools >= 45
-    rich
-    build
-    wheel
+extras=test
 commands=
     python -X warn_default_encoding -m pytest {posargs}