Disable pytest-services plugin in test to avoid worker_id conflict (#612)

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

tox.ini

@@ -42,7 +42,9 @@ commands =
   towncrier --version {posargs} --yes
 
 [pytest]
-addopts = -ra
+# pytest-services also defines a worker_id fixture, disable
+# it so they don't conflict with each other (#611).
+addopts = -ra -p no:pytest-services
 testpaths = testing
 
 [flake8]