set PIP_USE_PEP517 = False for tests

pip 19.1 otherwise complains about "editable mode is not supported for
pyproject.toml-style projects"

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