use older setuptools for python 3.6

Author: lopuhin

tox.ini

@@ -8,6 +8,12 @@ deps =
 commands =
     py.test --cov-report=term --cov-report= --cov=extruct {posargs:extruct tests}
 
+[testenv:py36]
+deps =
+    setuptools<58  # https://stackoverflow.com/a/69100830/217088
+commands =
+    pip install -r requirements-dev.txt
+    py.test --cov-report=term --cov-report= --cov=extruct {posargs:extruct tests}
 
 [testenv:py38]
 commands =