docker: stick to psycopg2 rather than psycopg2-binary

We've ran into the SCRAM authentication error thing on our Macbook
M1/2, so use the simplest workaround that seems to work:

psycopg/psycopg2#1360 (comment)

requirements.txt

@@ -4,7 +4,7 @@ python-dotenv==0.21.0
 pre-commit==2.20.0
 flake8==5.0.4
 black==22.8.0
-isort==5.12.0
-psycopg2-binary==2.9.5
+isort==5.10.1
+psycopg2==2.9.5
 django-referrer-policy==1.0
-django-csp==3.7
+django-csp==3.7