You don't need tox *inside* your test environment.

You need tox to *manage* your test environment (this is the premise, at least).

.circleci/setup-virtualenv.sh

@@ -28,6 +28,10 @@ shift || :
 export PIP_FIND_LINKS="file://${WHEELHOUSE_PATH}"
 export PIP_NO_INDEX="1"
 
+# Get tox inside the bootstrap virtualenv since we use tox to manage the rest
+# of the environment.
+"${BOOTSTRAP_VENV}"/bin/pip install "tox~=3.0"
+
 # Get everything else installed in it, too.
 "${BOOTSTRAP_VENV}"/bin/tox \
      -c "${PROJECT_ROOT}"/tox.ini \

setup.py

@@ -416,7 +416,6 @@ setup(name="tahoe-lafs", # also set in __init__.py
               "pip==22.0.3",
               "wheel==0.37.1",
               "setuptools==60.9.1",
-              "tox~=3.0",
               "subunitreporter==22.2.0",
               "python-subunit==1.4.2",
               "junitxml==0.7",