Run pytest with current directory in the search path

Without this, `pytest-django` cannot find its settings module.

https://docs.pytest.org/en/6.2.x/usage.html#calling-pytest-through-python-m-pytest

tox.ini

@@ -30,7 +30,7 @@ passenv =
     DB_*
 usedevelop = True
 commands =
-    pytest {posargs}
+    python -m pytest {posargs}
 
 [testenv:flake8]
 basepython =