diff --git a/tox.ini b/tox.ini
index ef543890f..d0e519ce4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -24,6 +24,11 @@ deps =
     pip>=10
 
 setenv =
+    # we have a pyproject.toml, but don't want pip to use it for building.
+    # (otherwise we get an error about 'editable mode is not supported for
+    # pyproject.toml-style projects').
+    PIP_USE_PEP517 = false
+
     PYTHONDONTWRITEBYTECODE = no_byte_code
     COVERAGE_PROCESS_START = {toxinidir}/.coveragerc