Avoid explicit pip install.

--- black-25.9.0/tox.ini.orig
+++ black-25.9.0/tox.ini
@@ -13,13 +13,14 @@
 recreate = True
 deps =
     -r{toxinidir}/test_requirements.txt
+extras =
+    d
+    jupyter
 commands =
-    pip install -e .[d]
     coverage erase
     pytest tests --run-optional no_jupyter \
         --numprocesses auto \
         --cov {posargs}
-    pip install -e .[jupyter]
     pytest tests --run-optional jupyter \
         -m jupyter \
         --numprocesses auto \