Remove default xdist argument to pytest

CONTRIBUTING.rst

@@ -73,7 +73,7 @@ You want to contribute some code? Great! Here are a few steps to get you started
       # install test dependencies
       (venv) $ pip install -U -r requirements/test.txt
       # run tests in the current environment, faster than tox
-      (venv) $ pytest --cov
+      (venv) $ pytest -n auto --cov
       # (optional) run tests for other python versions in separate environments
       (venv) $ tox
 

tox.ini

@@ -26,7 +26,7 @@ deps =
     -rrequirements/test.txt
 
 commands =
-    pytest --cov --cov-config .coveragerc --cov-append --cov-report="" {posargs}
+    pytest -n 2 --cov --cov-config .coveragerc --cov-append --cov-report="" {posargs}
 
 [testenv:lint]
 skip_install = true
@@ -46,7 +46,7 @@ commands =
 [pytest]
 DJANGO_SETTINGS_MODULE = testproj.settings.local
 python_paths = testproj
-addopts = -n 2 --ignore=node_modules
+addopts = --ignore=node_modules
 
 [flake8]
 max-line-length = 120