diff --git a/tox.ini b/tox.ini
index 24a5b0ff5..a1512a05d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -71,6 +71,7 @@ commands = {[testenv:py]commands}
 # tools/pinning/oldest/pyproject.toml.
 basepython = python3.8
 setenv = CERTBOT_OLDEST=1
+package = editable-legacy
 commands = {[testenv:py]commands}
 
 [testenv:cover]