Use --runpytest=subprocess to ensure tests run correctly

Author: pelme

setup.cfg

@@ -1,7 +1,7 @@
 [pytest]
 # --strict: warnings become errors.
 # -r fEsxXw: show extra test summary info for everything.
-addopts = --ignore lib/ --ignore build/ --ignore include/ --ignore local/ --ignore src/ --strict -r fEsxXw
+addopts = --ignore lib/ --ignore build/ --ignore include/ --ignore local/ --ignore src/ --strict -r fEsxXw --runpytest=subprocess
 DJANGO_SETTINGS_MODULE = pytest_django_test.settings_sqlite_file
 
 [wheel]