Fix pytest installation instructions
Fix pytest installation instructions

Since the removal of requirements_test.txt (in 2a0e8fb756d7c9dfce5f66e2b3cf68274dbc96d5 and 63610e0fd83d2fbb4126963bbe22b5b0a76f6104) it is no longer the case that `pip install -r requirements.txt` will install pytest.
tabo/django-treebeard,django-treebeard/django-treebeard,tabo/django-treebeard,tabo/django-treebeard,tabo/django-treebeard,django-treebeard/django-treebeard,django-treebeard/django-treebeard