Resolve "Updating own past feedback sets other tutors assignment to is_done=True"

Closes #155 (closed)

requirements.dev.txt

 flake8~=3.6.0
 pre-commit~=1.13.0
-pytest~=5.0.1
+pytest~=4.4
 pytest-cov~=2.6.0
 pytest-xdist~=1.29
 pytest-django~=3.5.0